avocado.utils.build: introduce env parameter to make()

This way users can specify environment variables (such as CFLAGS,
which is very common) to be used when building something using make.

Signed-off-by: Ademar de Souza Reis Jr <areis@redhat.com>
tests/doublefree.py: build with debug symbols

So that we can use it to showcase gdb support

Signed-off-by: Ademar de Souza Reis Jr <areis@redhat.com>
Merge pull request #201 from adereis/master

uitls.build.make(): support env variables (such as CFLAGS)