travis-ci: run `make check` in travis build
travis-ci: run `make check` in travis build

Test programs are only compiled when running `make check`.
Also, fix passing $CFLAGS argument to make.

Acked-by: Thomas Graf <17c2d8920621522552b392f72d8fdb4d1e866b44@suug.ch>
Signed-off-by: Thomas Haller <16e26612e4159990a6b3691e92592e691ff74132@redhat.com>

tklauser/libnl,honsys/libnl,claudelee/libnl,thom311/libnl,honsys/libnl,HolyShitMan/libnl-1,houstar/libnl,claudelee/libnl,greearb/libnl-ct,mshirley/libnl,kolyshkin/libnl,HolyShitMan/libnl-1,HolyShitMan/libnl-1,congwang/libnl,greearb/libnl-ct,dsahern/libnl,greearb/libnl-ct,congwang/libnl,thom311/libnl,mshirley/libnl,dsahern/libnl,cfriedt/libnl,houstar/libnl,cfriedt/libnl,tklauser/libnl,kolyshkin/libnl,tgraf/libnl,cfriedt/libnl,baloo/libnl,honsys/libnl,baloo/libnl,kolyshkin/libnl,congwang/libnl,dsahern/libnl,mshirley/libnl,mshirley/libnl,tklauser/libnl,claudelee/libnl,tgraf/libnl,kolyshkin/libnl,thom311/libnl,HolyShitMan/libnl-1,dsahern/libnl,houstar/libnl,tklauser/libnl,cfriedt/libnl,houstar/libnl,tgraf/libnl,congwang/libnl,tgraf/libnl,honsys/libnl,claudelee/libnl,baloo/libnl,baloo/libnl