Checkpoint commit for change over to CMake
Also remove m4 subdirectory, yay!
Bring build system more or less up to speed with previous autoconf-based
version
Get defines in order with prefix; also simplify endian detection a bit.
Enable all tests we can care about
The meta-config.h.in file now belongs to the sources, with CMake not
generating it automatically.
Fix requirements and build instructions
Generate meta.pc again
Add include guard
Fix generation of meta.pc, and installation of all files
Support Clang with compiler flags
Update travis to use cmake