Summary:
"./configure --target=TARGET" was broken; it would use the host gcc.
(So you had to explicitly specify "--with-gcc=TARGET-gcc" also,
as a workaround.)

This was broken by commit fc4856f9
for #8148. A comment claimed that FP_ARG_WITH_PATH_GNU_PROG_OPTIONAL
was the same as FP_ARG_WITH_PATH_GNU_PROG except for not raising
an error when the program isn't found; but that wasn't true --
the former didn't prepend the target name when cross-compiling.

We actually need three versions of FP_ARG_WITH_PATH_GNU_PROG since
the LLVM tools are usually not prefixed with the target name even
when cross-compiling. So I generalized the logic in a single macro.

Test Plan:
Built with "./configure --target=i386-unknown-linux"
and BuildFlavour=quick, successfully

Reviewers: ezyang, austin

Reviewed By: ezyang, austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D204