Double-build glitch, and repeated building forever on error if *.c and *.s are watched for changes

[H-A-M-G-E-R]

when i save a .c file in https://github.com/H-A-M-G-E-R/mf with "Rebuild on changes" on, it will build twice, and if the C code has an error it will try to build forever.

Target build directory is /build/asm, base is /build/src, no arguments in make, *.c and *.s are watched for changes

Fixes both of the above if *.s isn't watched for changes, so please exclude them in the default project settings.