configure: support --target-triples and --only-gcc.

Signed-off-by: Elly Jones <[email protected]>

configure

@@ -249,19 +249,10 @@ opt valgrind 1 "run tests with valgrind"
 opt docs     1 "build documentation"
 opt optimize 1 "build optimized rust code"
 opt mingw-cross 0 "cross-compile for win32 using mingw"
+opt only-gcc 0 "prefer gcc to clang for building the runtime"
 valopt prefix "/usr/local" "set installation prefix"
 valopt llvm-root "" "set llvm root"
-
-if [ $HELP -eq 1 ]
-then
-    echo ""
-    echo "Useful environment variables:"
-    echo ""
-    printf "    %-32s %s\n" "CFG_ONLY_GCC" "Prefer gcc to clang for building the runtime"
-    printf "    %-32s %s\n" "CFG_TARGET_TRIPLES" "LLVM target triples"
-    echo ""
-    exit 0
-fi
+valopt target-triples "" "LLVM target triples (defaults to host if unset)"
 
 step_msg "writing out basic parameters"
 putvar CFG_SRC_DIR