script was defaulting the LLVMGCC variable to "llvm-gcc" if it couldn't
find llvm-gcc and --with-llvmgccdir was not specified. In this case, there
is no llvm-gcc available on the system so we shouldn't assume that the
user's path will find it any better than configure could. The fix is to
default it to an empty string. If LLVMGCC is empty, the makefiles will
avoid building things that depend on llvm-gcc and give a nice warning
message to that effect.

llvm-svn: 21953