Make looking for multiple source files for a package more robust

* m4/gpg4win.m4: Do not check if _gpg4win_file is empty when checking
for matching files. Replace _gpg4win_suffix with a regular expression
matching all possible suffixes.

With this change all matching source files with all possible suffixes
are found. This prevents problems if the source files have different
suffixes. For example, I was bitten by having gpgme-1.18.1-beta46.tar.xz
and gpgme-1.19.0.tar.bz2 in packages. configure happily used the old
package and ignored the new one and made me doubt my sanity when gpg4win
didn't support features it should have supported.