Trac #28045: spkg-configure.m4 for libatomic_ops.

Implement a simple check for libatomic_ops >= 7.6.2 using pkg-config.
This suffices to detect all versions of the library available in
Gentoo linux.

Author: orlitzky

build/pkgs/libatomic_ops/spkg-configure.m4

@@ -0,0 +1,6 @@
+SAGE_SPKG_CONFIGURE([libatomic_ops], [
+  PKG_CHECK_MODULES([LIBATOMIC_OPS],
+                    [atomic_ops >= 7.6.2],
+                    [],
+                    [sage_spkg_install_libatomic_ops=yes])
+])