Moved the FLEX and BISON macros from Makefile.common since they are

configuration options like CC and CXX.
Updated LLVMGCCDIR so that it refers to the valid LLVM gcc code.
Added pathnames and flags to be used by some of the tests.
Moved configuration options from Makefile.common to here since they
should all be in one place.

llvm-svn: 6404