meson.build: Improve checks for va_copy() and __va_copy()

On Visual Studio, the compilation of the check program for va_copy() and
__va_copy() succeeds even though they may not be really available.  So,
make sure we include msvc_recommended_pragmas.h which helps us to detect
this situation by bailing out when warning C4013 (which means this
function is really not available) is encountered.

Also make sure that on Visual Studio builds we always include
msvc_recommended_pragmas.h is included so that it helps us to find out
problems in the build, and update comments for dirent.h and sys/time.h
as they are not shipped with any Visual Studio.

https://bugzilla.gnome.org/show_bug.cgi?id=783270