package/libglib2: fix compilation issue on SPARC

Fixes http://autobuild.buildroot.net/results/b3c/b3cb6515496cf09d305a1077d01f87d75ece8a8d/



Patch added to improve detection of atomic support in the compiler.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>