* build: add cross-compilation support to make
Set CROSS_COMPILE when running make to use the selected cross
compilation toolchain, such as arm-linux-gnueabihf, or
aarch64-linux-gnu.
Testing requires the presence of qemu - 'qemu-$(ARCH)' will be executed,
where ARCH is the first part of the toolchain triplet.
* build: add cross-compilation support to cmake
If C_COMPILER/CXX_COMPILER/CC/CXX are found to have cross-compilation
triplets in front of the compiler, then qemu will be used to execute the
tests.
* CI: add arm-linux-gnueabihf-gcc builder to Travis
The version of qemu available in Ubuntu trusty (as provided by Travis)
appears to have a bug in qemu-aarch64, which leads to the compatibility
tests failing on some inputs, erroneously rejecting the input as
corrupt.
Once Travis supports xenial, we could add an aarch64-gnu-linux-gcc
builder as well.
* CI: propagate cmake errors out of .travis.sh
Seems like even if cmake fails, the error isn't picked up by Travis.
