do not install all triplets

Hi,

I noticed ALL the triplets were installed, this fixes it, it only installs the symlinks _triplet_-clang[++] for which a _triplet_-gcc were found.

I did not know to write this cleanly with autotools, and I'm much conmfortable with cmake, as it remove the need for shell/m4/autoconf scripts scattered around, all the infrastructure is written in neat cmakelists.

Usage example and output:
```
[build] $ cmake -DCMAKE_INSTALL_PREFIX=$PWD/install ..
-- The C compiler identification is GNU 4.9.2
-- The CXX compiler identification is GNU 4.9.2
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Looking for getpid
-- Looking for getpid - found
-- Looking for unsetenv
-- Looking for unsetenv - found
-- Looking for include file sys/types.h
-- Looking for include file sys/types.h - found
-- Try C++11 flag = [-std=c++0x]
-- Performing Test CXX11_FLAG_DETECTED
-- Performing Test CXX11_FLAG_DETECTED - Success
-- Found CXX11: -std=c++0x  
-- Performing Test HAVE_STD_CHRONO
-- Performing Test HAVE_STD_CHRONO - Success
-- Found clang: /usr/bin/clang
-- Found mingw-gcc: /usr/bin/i686-w64-mingw32-gcc
-- Found mingw-gcc: /usr/bin/x86_64-w64-mingw32-gcc
-- Configuring done
-- Generating done
-- Build files have been written to: /home/xantares/projects/wclang/build


[xantares@arch64 build]$ make install
Scanning dependencies of target wclang
[ 33%] Building CXX object src/CMakeFiles/wclang.dir/wclang.cpp.o
[ 66%] Building CXX object src/CMakeFiles/wclang.dir/wclang_time.cpp.o
[100%] Building CXX object src/CMakeFiles/wclang.dir/wclang_cache.cpp.o
Linking CXX executable wclang
[100%] Built target wclang
Install the project...
-- Install configuration: ""
-- Installing: /home/xantares/projects/wclang/build/install/bin/wclang
-- Installing: /home/xantares/projects/wclang/build/install/bin/i686-w64-mingw32-clang
-- Installing: /home/xantares/projects/wclang/build/install/bin/i686-w64-mingw32-clang++
-- Installing: /home/xantares/projects/wclang/build/install/bin/x86_64-w64-mingw32-clang
-- Installing: /home/xantares/projects/wclang/build/install/bin/x86_64-w64-mingw32-clang++


[build]$ make package_source
Run CPack packaging tool for source...
CPack: Create package using TGZ
CPack: Install projects
CPack: - Install directory: /home/xantares/projects/wclang
CPack: Create package
CPack: - package: /home/xantares/projects/wclang/build/wclang-0.5.tar.gz generated.
CPack: Create package using TBZ2
CPack: Install projects
CPack: - Install directory: /home/xantares/projects/wclang
CPack: Create package
CPack: - package: /home/xantares/projects/wclang/build/wclang-0.5.tar.bz2 generated.
```


For discussion.