configure.ac: Check for qsort_r function before running the test program

configure.ac: Check for qsort_r function before running the test program

When cross compiling, test programs cannot be run. In order to make it
easier to cross compile for targets that do not have qsort_r(), check
for the function before trying to run the test program.

This avoid having to populate cache with glib_cv_have_qsort_r=no with
e.g. mingw cross compiler.

https://bugzilla.gnome.org/show_bug.cgi?id=651920