Require C++17 (#9481)

Summary:
Drop support for some old compilers by requiring C++17 standard
(or higher). See https://github.com/facebook/rocksdb/issues/9388

First modification based on this is to remove some conditional compilation in slice.h (also
better for ODR)

Also in this PR:
* Fix some Makefile formatting that seems to affect ASSERT_STATUS_CHECKED config in
some cases
* Add c_test to NON_PARALLEL_TEST in Makefile
* Fix a clang-analyze reported "potential leak" in lru_cache_test
* Better "compatibility" definition of DEFINE_uint32 for old versions of gflags
* Fix a linking problem with shared libraries in Makefile (`./random_test: error while loading shared libraries: librocksdb.so.6.29: cannot open shared object file: No such file or directory`)
* Always set ROCKSDB_SUPPORT_THREAD_LOCAL and use thread_local (from C++11)
  * TODO in later PR: clean up that obsolete flag
* Fix a cosmetic typo in c.h (https://github.com/facebook/rocksdb/issues/9488)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9481

Test Plan:
CircleCI config substantially updated.

* Upgrade to latest Ubuntu images for each release
* Generally prefer Ubuntu 20, but keep a couple Ubuntu 16 builds with oldest supported
compilers, to ensure compatibility
* Remove .circleci/cat_ignore_eagain except for Ubuntu 16 builds, because this is to work
around a kernel bug that should not affect anything but Ubuntu 16.
* Remove designated gcc-9 build, because the default linux build now uses GCC 9 from
Ubuntu 20.
* Add some `apt-key add` to fix some apt "couldn't be verified" errors
* Generally drop SKIP_LINK=1; work-around no longer needed
* Generally `add-apt-repository` before `apt-get update` as manual testing indicated the
reverse might not work.

Travis:
* Use gcc-7 by default (remove specific gcc-7 and gcc-4.8 builds)
* TODO in later PR: fix s390x "Assembler messages: Error: invalid switch -march=z14" failure

AppVeyor:
* Completely dropped because we are dropping VS2015 support and CircleCI covers
VS >= 2017

Also local testing with old gflags (out of necessity when using ROCKSDB_NO_FBCODE=1).

Reviewed By: mrambacher

Differential Revision: D33946377

Pulled By: pdillinger

fbshipit-source-id: ae077c823905b45370a26c0103ada119459da6c1

INSTALL.md

@@ -6,7 +6,7 @@ than release mode.
 
 RocksDB's library should be able to compile without any dependency installed,
 although we recommend installing some compression libraries (see below).
-We do depend on newer gcc/clang with C++11 support.
+We do depend on newer gcc/clang with C++17 support (GCC >= 7, Clang >= 5).
 
 There are few options when compiling RocksDB:
 
@@ -50,7 +50,7 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
 ## Supported platforms
 
 * **Linux - Ubuntu**
-    * Upgrade your gcc to version at least 4.8 to get C++11 support.
+    * Upgrade your gcc to version at least 7 to get C++17 support.
     * Install gflags. First, try: `sudo apt-get install libgflags-dev`
       If this doesn't work and you're using Ubuntu, here's a nice tutorial:
       (http://askubuntu.com/questions/312173/installing-gflags-12-04)
@@ -62,8 +62,7 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
     * Install zstandard: `sudo apt-get install libzstd-dev`.
 
 * **Linux - CentOS / RHEL**
-    * Upgrade your gcc to version at least 4.8 to get C++11 support:
-      `yum install gcc48-c++`
+    * Upgrade your gcc to version at least 7 to get C++17 support
     * Install gflags:
 
               git clone https://github.com/gflags/gflags.git
@@ -113,11 +112,11 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
               make && sudo make install
 
 * **OS X**:
-    * Install latest C++ compiler that supports C++ 11:
+    * Install latest C++ compiler that supports C++ 17:
         * Update XCode:  run `xcode-select --install` (or install it from XCode App's settting).
         * Install via [homebrew](http://brew.sh/).
             * If you're first time developer in MacOS, you still need to run: `xcode-select --install` in your command line.
-            * run `brew tap homebrew/versions; brew install gcc48 --use-llvm` to install gcc 4.8 (or higher).
+            * run `brew tap homebrew/versions; brew install gcc7 --use-llvm` to install gcc 7 (or higher).
     * run `brew install rocksdb`
 
 * **FreeBSD** (11.01):
@@ -160,7 +159,7 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
 
     * Install the dependencies for RocksDB:
 
-        pkg_add gmake gflags snappy bzip2 lz4 zstd git jdk bash findutils gnuwatch 
+        pkg_add gmake gflags snappy bzip2 lz4 zstd git jdk bash findutils gnuwatch
 
     * Build RocksDB from source:
 
@@ -182,13 +181,13 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
 * **Windows**:
   * For building with MS Visual Studio 13 you will need Update 4 installed.
   * Read and follow the instructions at CMakeLists.txt
-  * Or install via [vcpkg](https://github.com/microsoft/vcpkg) 
+  * Or install via [vcpkg](https://github.com/microsoft/vcpkg)
        * run `vcpkg install rocksdb:x64-windows`
 
 * **AIX 6.1**
     * Install AIX Toolbox rpms with gcc
     * Use these environment variables:
-  
+
              export PORTABLE=1
              export CC=gcc
              export AR="ar -X64"
@@ -199,9 +198,9 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
              export LIBPATH=/opt/freeware/lib
              export JAVA_HOME=/usr/java8_64
              export PATH=/opt/freeware/bin:$PATH
-  
+
 * **Solaris Sparc**
-    * Install GCC 4.8.2 and higher.
+    * Install GCC 7 and higher.
     * Use these environment variables:
 
              export CC=gcc
@@ -210,4 +209,3 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
              export EXTRA_LDFLAGS=-m64
              export PORTABLE=1
              export PLATFORM_LDFLAGS="-static-libstdc++ -static-libgcc"
-