Make KLEE fail in case main function is missing

Existence of main() function is checked with assertion.
This check fails if KLEE is compiled in Release mode.
Fix error message for failing linking of libraries

In case linking of external libraries failed, user would
only be informed if KLEE is compiled with assertions enabled.

This fix lets KLEE always fail.
Merge pull request #94 from MartinNowack/fix_assert_library_linking

Fix assert library linking