Warn if gcc version is lower than 4.8

[adamatan]

The -Og parameter was introduced in in gcc 4.8. When running make with older versions of gcc (as the default one in OSX/XCode), the following error message appears:

error: invalid integral value 'g' in '-Og'

The following PR shows an error message if the gcc version is older than 4.8. I've chosen not to exit the make process, just in case this logic would fail in some edge case.

See also:

• Makefile: Emit warning gcc version is lower than 4.8.0
• GCC 4.8 Release Series Changes, New Features, and Fixes

[dvirsky]

Actually you can remove -Og completely, but thanks for noticing this.