Add a way to limit deserialized string lengths

and use it for most strings being serialized.
Merge pull request #4655

216e9a4 Add a way to limit deserialized string lengths (Pieter Wuille)
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.
Add travis CI builds.
Set-up for multi compiler builds.
Add make check.