remove HAVE_GCC_PRAGMA_DIAGNOSTIC_MODIFY check and macro

This has been supported by GCC and clang for a very long time.

Change-Id: I410a2b39004c932003f8cbefe935aedb109b1163
remove HAVE_GCC_PRAGMA_DIAGNOSTIC_SCOPE check and macro

This is supported in GCC 4.6.0 already:
https://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Diagnostic-Pragmas.html

Change-Id: I2f67e588eea3a323a2e9c81e39e56ab2e715a817
remove checks of __GCC_MINOR__ that are always true in GCC 4.6 or later

Change-Id: I4412a0c1e4dee94cd717f24b8df5e257d895f721
MWERKS etc. are not supported toolchains

Change-Id: I449fe34be396114529357388c87162aaf8976292