build: port to GCC 4.6.4 + glibc 2.5

build: port to GCC 4.6.4 + glibc 2.5

On platforms this old, building with _FORTIFY_SOURCE equal to 2
results in duplicate definitions of standard library functions.
Problem reported by Nelson H. F. Beebe.
* configure.ac (_FORTIFY_SOURCE): Sort after GNULIB_PORTCHECK.
By default, do not enable this unless GNULIB_PORTCHECK is defined.
This better matches the original intent, which as I recall was to
enable these extra checks only with --enable-gcc-warnings.