build: Move m4 scripts (incl. auto-generated m4s) out of src.

Create build-aux/m4 dir. Move m4 scripts for out-of-src builds.
Tidy away pkg.m4 script. Readibility and fixes AC_CONFIG macro
calls in configure.ac. Add warnings for autoreconf.

Move m4 to standard location. 
Beginning of out-of-src build work but useful on its own.
Tested on linux RH-derivative, ubuntu and OSX 10.7. Needs further cross-platform testing.