Build System: Strip '-std=c99' from CFLAGS provided by libraries.

Asterisk requires GNU C extensions.  On some systems certain libraries
may incorrectly push -std=c99 into CFLAGS, thus breaking the build.
This change causes that flag to be stripped so the Asterisk build is not
broken by those libraries.  This change is made for both pkgconfig and
tool based libraries.

ASTERISK-27629 #close

Change-Id: I13389613b194abbac77becf90cd950dc168704db