Augment gcc's debugging capabilities

Note: icc just ignores the function, and thus falls back on -O0, which
should be fine enough.

Thanks PAW!

configure.ac

@@ -1944,6 +1944,10 @@ if test x$enable_debug = xyes; then
 	FCFLAGS="$FCFLAGS -O0"
 	enable_spinlock_check=yes
 	if test x$GCC = xyes; then
+		CFLAGS="$CFLAGS -Og"
+		CXXFLAGS="$CXXFLAGS -Og"
+		FFLAGS="$FFLAGS -Og"
+		FCFLAGS="$FCFLAGS -Og"
 		if test x$starpu_windows != xyes ; then
 			if test x$enable_fstack_protector_all = xyes ; then
 			   CFLAGS="$CFLAGS -fstack-protector-all"