AC_PATH_TOOL breaks the ability to manually specify CC/CXX at configure time

In configure.ac, we use AC_PATH_TOOL to set variables like CC and CXX. Consider the example of AC_PATH_TOOL([CC],[gcc], [clang]), when both gcc and clang is in PATH and I'd like to manually specify CC=clang, it doesn't work at all, because AC_PATH_TOOL discards the already set variable!

This essentially breaks the ability to manually specify tools when not cross compiling, not just CC/CXX, but also AR, NM, etc. A rather unfortunate fallout of !8504 (closed).