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For Linux build testing, we want all targets compiled. To do so
without a cross-toolchain, simply use the host compiler for the
target tools. This can be achieved by setting CROSS_COMPILE to
an empty string.

OSX can't handle this: It neither supports "-static", nor would
it successfully compile meminfo.c. Thus we keep the default
"make", which should only build the 'tools' target.

On Linux we'll also test the install-* targets by requesting
installation to /tmp/sunxi-tools as a post-build step.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

After eliminating all warnings, we can disallow them for future
builds. This is done by passing an additional "-Werror" flag
from the Travis build step. Introducing new warnings will cause
a (CI) build failure from now on.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

We'll be testing both gcc and clang builds on Linux. As for now,
Travis CI only supports clang for OSX - so exclude gcc currently.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>