New CMake module to locate libconfig++

The current code to locate libconfig++ has some trouble:
- It is located in the main CMakeList file but should be a module of its own
- It depends on pkg-config which is not available on Windows
- The returned values are not really useful therefore the library name is hard coded

This commit tries to solve these problems. For a Unix-like system there should be no difference.
Additionally, you can override the variables on the command line which is useful for builds on Windows.

cmake/Modules/FindLibConfig++.cmake

@@ -0,0 +1,24 @@
+# Find the libconfig++ includes and library
+#
+# This module defines
+# LIBCONFIG++_INCLUDE_DIR, where to find libconfig++ include files, etc.
+# LIBCONFIG++_LIBRARy, the library to link against to use libconfig++.
+# LIBCONFIG++_FOUND, If false, do not try to use libconfig++.
+
+# also defined, but not for general use are
+# LIBCONFIG++_LIBRARY, where to find the libconfig++ library.
+
+set(LIBCONFIG++_FOUND TRUE)
+
+find_path(LIBCONFIG++_INCLUDE_DIR libconfig.h++)
+
+find_library(LIBCONFIG++_LIBRARY config++)
+
+if (LIBCONFIG++_INCLUDE_DIR AND LIBCONFIG++_LIBRARY)
+   set(LIBCONFIG++_FOUND TRUE)
+endif (LIBCONFIG++_INCLUDE_DIR AND LIBCONFIG++_LIBRARY)
+
+# Use the default CMake facilities for handling QUIET/REQUIRED.
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LLVM
+    REQUIRED_VARS LIBCONFIG++_INCLUDE_DIR LIBCONFIG++_LIBRARY)