Use CMake variables for paths in pkg-config files

Use @gRPC_INSTALL_LIBDIR@ for libdir; this fixes an incorrect
-L/usr/lib on multilib Linux systems where that is the 32-bit library
path and the correct path is /usr/lib64.

Use @gRPC_INSTALL_INCLUDEDIR@ for consistency.

cmake/pkg-config-template.pc.in

@@ -1,7 +1,7 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-includedir=${prefix}/include
-libdir=${exec_prefix}/lib
+includedir=@gRPC_INSTALL_INCLUDEDIR@
+libdir=@gRPC_INSTALL_LIBDIR@
 
 Name: @PC_NAME@
 Description: @PC_DESCRIPTION@