curl-compilers.m4: disable warning spam with Cygwin's clang

When building with Cygwin or MinGW, libtool uses a wrapper executable
instead of a wrapper script [1], which is written in C and throws
missing-variable-declarations warnings. Don't enable these warnings on
Cygwin and MinGW in order to avoid warnings for every executable built,
which spams the test suite output when using Cygwin's clang.

[1] https://www.gnu.org/software/libtool/manual/html_node/Wrapper-executables.html

Closes https://github.com/curl/curl/pull/1665