Change default C compiler flags
Change default C compiler flags

O1 and O2 have indistinguishable performance (for me) but O1 compiles
far quicker. Also include -foptmizie-sibling-calls (it's included in O1
in clang, but not gcc, as far as I can tell).

Please shout if this turns out to break anything...

kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev