(CMAKE ROSE) add cc to list of C compilers

This change allows testqualifiedName to pass on the CMake build
system.  Without cc in the list of C compilers recognized by
create_system_headers, the C compiler include-staging directory
is left unpopulated.
(CMAKE ROSE) setup C/CXX_INCLUDE_STRING properly

This resolves issue #23.

We now call get_compiler_header_dir to generate an apporpriate
list of include directories for our C & C++ compilers.  This value
is stored in rose_config.h, as done by the autoconf build.
(CMAKE ROSE) bump minimum CMake version to 2.8

This minimum required version bump will provide a clearer
error message on systems running CMake 2.6 or older.
This change also requird me to fix a problem where the
same source directory (PHPFrontend) was being added twice.
Merge branch 'cmake_on_linux' of https://github.com/zackgalbreath/rose into cmake_on_linux