Specify -latomic_ops as private in pkg-config file

(fix of commit 02a44ee)

PR #655 (bdwgc).

Actually, bdwgc itself may requires atomic_ops, but it does not require
downstream packages (like guile) to require atomic_ops.  Now we specify
@ATOMIC_OPS_LIBS@ in Libs.private rather than in Libs, so that will
not confuse downstream packages, especially when performing check.

* bdw-gc.pc.in (Libs.private): Move @ATOMIC_OPS_LIBS@ from Libs.

bdw-gc.pc.in

@@ -6,5 +6,6 @@ includedir=@includedir@
 Name: Boehm-Demers-Weiser Conservative Garbage Collector
 Description: A garbage collector for C and C++
 Version: @PACKAGE_VERSION@
-Libs: -L${libdir} @ATOMIC_OPS_LIBS@ -lgc @THREADDLLIBS@
+Libs: -L${libdir} -lgc @THREADDLLIBS@
+Libs.private: @ATOMIC_OPS_LIBS@
 Cflags: -I${includedir}