Provide configured include/library paths and private static-link dependencies.

--- lib/libnbd.pc.in.orig	2023-11-16 10:50:20 UTC
+++ lib/libnbd.pc.in
@@ -24,6 +24,7 @@ Description: NBD client library in userspace
 Name: @PACKAGE_NAME@
 Version: @PACKAGE_VERSION@
 Description: NBD client library in userspace
-Requires:
-Cflags:
-Libs: -lnbd
+Requires.private: @LIBNBD_REQUIRES_PRIVATE@
+Cflags: -I${includedir}
+Libs: -L${libdir} -lnbd
+Libs.private: @PTHREAD_LIBS@