Modernize makefiles

Modernize makefiles in sc_lib_fixed_point.

My motivation is that currently the top level Makefile have a hardcoded list of apps to build (app_example_fixed_point). When sc_lib_fixed_point is pulled in from another app without copying this app running make all in sc_lib_fixed_point fails. Setting BUILD_SUBDIRS to all fixes this as it only builds the apps that are actually there.