* don't assume includedir

This change syncs the public header include install location with what gets written into pkgconfig file.

CMakeLists.txt

@@ -531,4 +531,5 @@ if (UNIX OR MINGW OR CYGWIN)
     install(FILES ${PROJECT_BINARY_DIR}/json-c.pc DESTINATION "${INSTALL_PKGCONFIG_DIR}")
 endif ()
 
-install(FILES ${JSON_C_PUBLIC_HEADERS} DESTINATION ${CMAKE_INSTALL_PREFIX}/include/json-c)
+install(FILES ${JSON_C_PUBLIC_HEADERS} DESTINATION ${CMAKE_INSTALL_FULL_INCLUDEDIR}/json-c)
+