configure.ac: Drop LLVM compiler flags more radically

configure.ac: Drop LLVM compiler flags more radically

Drop all -m*, -W*, -O*, -g* and -f* flags, with the exception of
-fno-rtti, which must be used if it's part of the llvm-config --cxxflags
output. We don't want LLVM to dictate the flags we use, and it can even
cause build failures, e.g. if LLVM and Mesa are built with different
compilers.

While we're at it, eat any whitespace preceding dropped flags as well.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>