pkg-config: support INCLUDE_INSTALL_DIR
diff --git a/pkg-config/jsoncpp.pc.in b/pkg-config/jsoncpp.pc.in
index d8a61ac..5de8105 100644
--- a/pkg-config/jsoncpp.pc.in
+++ b/pkg-config/jsoncpp.pc.in
@@ -1,7 +1,7 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
 libdir=${exec_prefix}/lib
-includedir=${prefix}/include
+includedir=${prefix}/@INCLUDE_INSTALL_DIR@
 
 Name: jsoncpp
 Description: A C++ library for interacting with JSON