In 897d68e (Makefile: use curl-config --cflags, 2020-03-26), we
taught the build process to use `curl-config --cflags` to make sure that
it can find cURL's headers.

In the MSVC build, this is completely bogus because we're running in a
Git for Windows SDK whose `curl-config` supports the _GCC_ build.

Let's just ignore each and every `-I<path>` option where `<path>` points
to GCC/Clang specific headers.

Reported by Jeff Hostetler in
microsoft#275.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>