Remove redundant checks for standard library functions

The single-threaded CMake configuration step takes an excessive amount of time because of redundant checks such as the following:

CHECK_FUNCTION_EXISTS (vsnprintf HAVE_VSNPRINTF)

As far as I understand, vsnprintf was introduced in ISO/IEC 9899:1999 a.k.a. C99.

All such checks cost time, because the compiler needs to be invoked to attempt the compilation of a test program.

On Microsoft Windows, a hack in cmake/os/WindowsCache.cmake is used to pre-record the answers of many configuration checks. If we don't dare to remove some checks in the fear that it might break the build on some operating system that we do not have CI coverage of, such as DragonflyBSD or Illumos, we might add a cmake/os/LinuxCache.cmake that we'd include from cmake/os/Linux.cmake.