makefile.m32: allow to override gcc, ar and ranlib

Allow to ovverride certain build tools, making it possible to
use LLVM/Clang to build curl. The default behavior is unchanged.
To build with clang (as offered by MSYS2), these settings can
be used:

CURL_CC=clang
CURL_AR=llvm-ar
CURL_RANLIB=llvm-ranlib

Closes https://github.com/curl/curl/pull/1993