Always expose the use_libc++ option. It's only useful for Clang but cmake.class
tries to find when we've used CMake options which are unused/unknown and this
triggers it for GCC builds if we set -Duse_libc++=OFF.

So, always expose the option so we can turn it off without a warning.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -21,7 +21,7 @@ option(build_xmlparser "Automatically update the XML parser modules when updatin
 option(build_search    "Build external search tools (doxysearch and doxyindexer)" OFF)
 option(build_doc       "Build user manual (HTML and PDF)" OFF)
 option(build_doc_chm   "Build user manual (CHM)" OFF)
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+if (TRUE)
     option(use_libc++  "Use libc++ as C++ standard library." ON)
 endif()
 option(use_libclang    "Add support for libclang parsing." OFF)
