Print message when libconfig++ is not found.

I only made it a warning for now, because there is presumably a reason why the package isn't marked required (pkgconfig-less installations maybe?).

CMakeLists.txt

@@ -21,6 +21,7 @@ else(NOT PKG_CONFIG_FOUND)
     if(NOT LIBCONFIGPP_FOUND)
         set(LIBCONFIG_CXXFLAGS "" CACHE STRING "libconfig++ compiler flags")
         set(LIBCONFIG_LDFLAGS "" CACHE STRING "libconfig++ linker flags")
+        message(WARNING "libconfig++ not found. If compilation fails but the library is installed, consider manually setting the LIBCONFIG_CXXFLAGS and LIBCONFIG_LDFLAGS CMake variables.")
     else(NOT LIBCONFIGPP_FOUND)
         set(LIBCONFIG_CXXFLAGS ${LIBCONFIGPP_CFLAGS} CACHE STRING "libconfig++ compiler flags")
         set(LIBCONFIG_LDFLAGS ${LIBCONFIGPP_LDFLAGS} CACHE STRING "libconfig++ linker flags")