Fix build with DEBUG_LOCKORDER

Fixes #4771
qt/splashscreen: #include version.h

Needed to build breakage reported by Arnavion on IRC:
qt/splashscreen.cpp: In constructor 'SplashScreen::SplashScreen(const QPixmap&, Qt::WindowFlags, bool)':
qt/splashscreen.cpp:33:98: error: 'FormatFullVersion' was not declared in this scope
Create build-aux/m4 dir. Move m4 scripts for out-of-src builds.
Remove pkg.m4. Update .gitignore.