This change is done because using strongly-typed C++11 enums, prefixes are removed from enum items
(as they're scoped) and that has a big tendency to clash with generated parser defines.
Windows build untested.
Use FB_GEN_DIR env var. to determin location of generated output.
Tidy up usage of FB_GEN_DB_DIR - only use with unix style paths.
Generate platform specific log files during build process.
Refactor common code (mostly into set_build_target.bat).
Ensure that all VS2005 x64 output is clearly generated into x64
directories, and likewise for Win32 output