gallium/drivers: compact compiler flags into Automake.inc

* minimise flags duplication
* distingush between VISIBILITY C and CXX flags
* set only required flags - C and/or CXX

v2: add LLVM_CFLAGS back to AM_CFLAGS (add missing backslash)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
17 files changed