add USER_LIBS to common.mk

[briantilley]

This is to solve issue #376; libraries can now be added to a project with the addition of USER_LIBS=... to the Makefile without removing those from parameters.mk.

[vlad-ivanov-name]

Wouldn't the following do the same (libm is used as example)?

PROGRAM=test

include ../../common.mk

LIBS+=m

[briantilley]

Yes, the end effect is the same. To me, at least, having a USER_LIBS variable makes the solution clearer when trying to add a library.

[UncleRus]

I believe that EXTRA_LIBS will be better name.