Use LIBSUBDIR for vpx.pc.

Change-Id: Ibba635696e8c23086e5d3c0e8452ab97c460d7d7

libs.mk

@@ -234,7 +234,7 @@ vpx.pc: config.mk libs.mk
 	$(qexec)echo '# pkg-config file from libvpx $(VERSION_STRING)' > $@
 	$(qexec)echo 'prefix=$(PREFIX)' >> $@
 	$(qexec)echo 'exec_prefix=$${prefix}' >> $@
-	$(qexec)echo 'libdir=$${prefix}/lib' >> $@
+	$(qexec)echo 'libdir=$${prefix}/$(LIBSUBDIR)' >> $@
 	$(qexec)echo 'includedir=$${prefix}/include' >> $@
 	$(qexec)echo '' >> $@
 	$(qexec)echo 'Name: vpx' >> $@