Pebble SDK uses waf as its build tool. It is configured in the wscript
file.
If you look at the wscript
file of your Pebble project you will see the following lines:
def build(ctx):
ctx.load('pebble_sdk')
ctx.pbl_program(source=ctx.path.ant_glob('src/**/*.c'),
target='pebble-app.elf')
This tells the compiler to compile and link together all the ".c" files in src/
and all of its sub-directories.