commmon: allow appending extra CFLAGS/LDFLAGS via command line

With this change, user may provide the extra flags CFLAGS/LDFLAGS using
the following syntax:

Example #1:
> make EXTRA_CFLAGS=-finstrument-functions <target>

Example #2:
> CFLAGS=-finstrument-functions make <target>
Merge pull request #54 from krzycz/common-makefile_extra_cflags

common: allow appending extra CFLAGS/LDFLAGS via command line