Add Pthreads (and possibly other) flags to the pkg-config file (#332)

Proper Libs.private enables linking applications statically against
libqb: static archives (.a) don't carry their own dependency
information, unlike shared libraries (.so).  Modern libc versions
include socket and RT functions, so socket_LIBS and rt_LIBS will be
empty there, but we include them for strict correctness on older
platforms; basically, we're matching libqb_la_LIBADD here.
Consequently, nsl_LIBS and GLIB_LIBS don't enter this field, since they
are only used in the examples and tests, not in the library proper.

Cflags, on the other hand, is emitted all the time and (under GCC)
propagates the -pthread option (which also affects the preprocessing
stage) to all users of libqb even when compiling modules or linking
everything dynamically.

Signed-off-by: Ferenc Wágner <[email protected]>

lib/libqb.pc.in

@@ -8,5 +8,5 @@ Version: @PACKAGE_VERSION@
 Description: libqb
 Requires:
 Libs: -L${libdir} -lqb
-Libs.private: @LIBS@ @SYSTEMD_LIBS@
-Cflags: -I${includedir}
+Libs.private: @LIBS@ @dlopen_LIBS@ @PTHREAD_LIBS@ @socket_LIBS@ @rt_LIBS@ @SYSTEMD_LIBS@
+Cflags: -I${includedir} @PTHREAD_CFLAGS@