Change @libdir@ in the pkg-config file to be $(usrlibdir) The variable in the .pc file points to the directory where the .so symlink is present, which is never going to be something like /lib, but something like /usr/lib.
diff --git a/Makefile b/Makefile
index df8e4fe..53ab4f3 100644
--- a/Makefile
+++ b/Makefile
@@ -112,7 +112,7 @@ $(LIB_PKGCONFIG): $(LIB_PKGCONFIG).in
sed -e 's:@VERSION@:$(LIB_VERSION):' \
-e 's:@prefix@:$(value prefix):' \
-e 's:@exec_prefix@:$(value exec_prefix):' \
- -e 's:@libdir@:$(value libdir):' \
+ -e 's:@libdir@:$(value usrlibdir):' \
-e 's:@includedir@:$(value includedir):' \
$< > $@