Buildfix and warningfix on gcc/clang.

This commit partially fixes the build for me but I didn't run any test yet (in fact I'm wondering how you can develop on such a broken build). The proper invocation for cmake will now be:

    cmake -DCMAKE_BUILD_TYPE=Debug <path_to_your_source>

and replace Debug with Release for a release build (I'm going to spend more time on that in the future and make it default to release if you don't pass any -DCMAKE_BUILD_TYPE parameter).

The only build problem I've got left is:

    Linking CXX executable compiler_basic
    /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.3/../../../../lib64/crt1.o: nella funzione "_start":
    (.text+0x20): riferimento non definito a "main"
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    make[2]: *** [test/frontend/compiler_basic] Errore 1
    make[1]: *** [test/frontend/CMakeFiles/compiler_basic.dir/all] Errore 2
    make: *** [all] Errore 2

test/frontend/compiler_basic.cpp has no main(), but I'm not sure if that's intentional. If it's not, and a main() should really be there, what would its body look like? I'm not familiar with the boost testing suite.