Merge pull request open-source-parsers#70 from jmesmon/pkg-config-include-var

pkg-config: support INCLUDE_INSTALL_DIR

Author: cdunn2001

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