build: travis: Install dependencies before build

In order to make sure all tools (except curvetun) are built by Travis
CI, install the libraries they depend upon.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: travis: Pass on CC environment variable to make

If travis compiles with clang, the environment variable CC is set. Since
the netsniff-ng build system doesn't currently pick it up unless
explicitly specified as a make variable, pass it on as such.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: travis: Disable clang for now

clang doesn't understand all our (GCC-spcific) compiler flags. We need
to work around it by filtering them out properly in case clang is used.
For now, just disable clang on travis, until I figured this out locally.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>