Fix inclusion of FlexLexer.h

1. Line 64 of preprocessor/Makefile.am reads cp /usr/include/FlexLexer.h .. Needs to be changed to include from directory where $(LEX) is installed.

2. Consider applying a patch for OSX 10.7, which has a bug in FlexLexer.h https://gist.github.com/houtanb/8268790