Fix build on Linux with mainstream gcc

[qu1ck]

3 fixes:

• Add .gitignore for build directory.
• Most non-windows filesystems differentiate case in file names so eeprom.h with lowercase breaks build on those filesystems.
• -fpromote-loop-indices is not in the mainstream gcc and the optimisation it is supposed to do should be done automatically so this flag is obsolete and also breaks build.