Summary:
Some compilers require `-std=c++11` for the `cstdint` header to be available. We already have logic to add `-std=c++11` to `CXXFLAGS` when the compiler is not MSVC; simply reorder CMakeLists.txt so that logic happens before the calls to `CHECK_CXX_SOURCE_COMPILES`.

Additionally add a missing `set(CMAKE_REQUIRED_FLAGS, ...)` before a call to `CHECK_C_SOURCE_COMPILES`.
Closes https://github.com/facebook/rocksdb/pull/2535

Differential Revision: D5384244

Pulled By: yiwu-arbug

fbshipit-source-id: 2dbae4297c5d8ab4636e08b1457ffb2d3e37aef4