Cleanup PackageInfo.g
Add GPL headers to source files
Cleanup configure.ac

In particular, remove various unneeded and/or obsolete macro invocations
Fix 'make check' hang when SingularInterface fails to load
Rewrite build system to use single Makefile.am

Also made the build rules silent by default, and various other tweaks
Silence some clang warnings about 'no previous prototype for function'