Improve runtime library search
* m4/gpg4win.m4 (GPG4WIN_RUNTIME_LIBRARY): Extend to allow optional runtime libs. (GPG4WIN_RUNTIME_LIBRARY_X64): New. 64bit variant of the RUNTIME_LIBRARY function. * configure.ac: Add optional x64 winpthread lookup. Improve check for for exception libs.
Since I also use a pthreads based variant mingw for x64 I
had to locally carry a patch that included this. Now with
the optional libs this should work robustly for anyone.