Fixes git builds w/o asciidoc, bug introduced in 46824abe

We only care about libstdc++ when using GNU C++.
Further, libstdc++ depends on libm on some platforms, fix this.

This fixes the build_doc logic to work as advertised and never
try to (re)build documentation when working with a tarball release.

It turns out that feature test macros are a complete mess, and defining
_GNU_SOURCE and _POSIX_C_SOURCE together breaks some systems. So we try
and define the appropriate "make everything visible" combination specific
to each system instead. This may still break something, so people should
test their platforms and speak up if so.

This should fix more exotic POSIX platforms such as HPUX which insist on
having _POSIX_C_SOURCE defined to get proper threading behaviour. We also
define _GNU_SOURCE so that we get any extensions on systems with glibc, e.g.
eventfd.