build: avoid --enable-gcc-warnings on GCC <= 4.5

build: avoid --enable-gcc-warnings on GCC <= 4.5

* configure.ac: Only enable warnings automatically when
on GCC >= 4.6 (and when building from a git checkout)
as that was the first GCC version to support fine-grained
control of warnings, allowing them to be adjusted around
certain code sections.  gnulib relies on this for certain
warnings, so avoid auto enabling this option lest we trigger
build failures on now over two year old compilers.
Reported by Zartaj Majeed with GCC 4.5.3 on cygwin.