pkg-config: Only append -lm if necessary.

Some systems don't require a separate -lm to link with libc
math functions. The configure script checks for this, so
use the corresponding variable to set the dependent libraries
in vorbis.pc dynamically.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>

vorbis.pc.in

@@ -11,5 +11,5 @@ Version: @VERSION@
 Requires.private: ogg
 Conflicts:
 Libs: -L${libdir} -lvorbis 
-Libs.private: -lm
+Libs.private: @VORBIS_LIBS@
 Cflags: -I${includedir}