Compiling C++ code with -D_POSIX_SOURCE on Solaris is unsupported, so remove it.
Isolate GCC-isms inside checks that we are actually using GCC/G++.
Only check for -lstdc++ when on GCC and doing static linking.

Removed README.source, TODO.source since these are irrelevant to a generic
git package.
Fixed spelling in debian/copyright.
Removed RFC check in debian/rules, again irrelevant to a generic git package.

Don't fail hard if an unreleased tarball is being built and asciidoc is
not installed; instead just print a big fat warning