Added Travis CI config file
MINGW: config.mak.uname: add explicit way to request MinGW-build

When crosscompiling, one cannot rely on `uname` from host system.
Thus, add an option to use `make MINGW=1` for building MinGW build
from non-MinGW host (Linux, for example).
MINGW: compat/bswap.h: include stdint.h

bswap.h uses uint32_t type which might not be defined.
This patch adds direct include so bswap.h can be safely included.
MINGW: compat/mingw.h: do not attempt to redefine lseek on mingw-w64

mingw-w64 has lseek defined in io.h.
Makefile: introduce CROSS_COMPILE variable

To ease cross-compilation process, introduce a single variable
with the prefix to all compiler-related executables.