Add a BUILD_FLAGS variable so that autoconf checks have a place

Add a BUILD_FLAGS variable so that autoconf checks have a place
to store additional flag options since too many things can
and do override CPPFLAGS. Also, this is exported, unlike CPPFLAGS
so it can be actually used elsewhere. This should enable us
to remove the AC_SUBSTs in the intel checks, but I have no way
of testing it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161233 91177308-0d34-0410-b5e6-96231b3b80d8