Fixed make parallelization bug with apr.h.

If starting with a pristine git checkout (thus no
3rdparty/apr/include/apr.h generated yet), parallel building ("make -j
N" where N > 1) used to fail.
Merge pull request #24 from mendel/fixing_make_parallelization_bug

Fixed make parallelization bug with apr.h.
Validate literal coderef

Based on bytecode.c:create_code_objects(), there is one coderef object
per frame, so check the given number against the number of frames.
Merge pull request #25 from Benabik/master

Validate coderef literal
Missing MVMROOT; JimmyZ++.
Make the build environment detection more generic
Add --clang option to Configure.pl

This forces usage of clang compiler over gcc if clang is found