backport -D flags and more p2 enhancements

-Llibdir
-DitpvGJ debug flags
--check
-DG GC_CHECK

enable -fno-omit-frame-pointer
compile only one vm target into the executable, no cross-compiler support.
add potion_dump helper with -DDEBUG
better potion_dump_stack
add runtime tracing with --bytecode
add parser tracing with -Dp (not yet)
backport from p2: no asan checks for stack walker

decr the stack is asan unsafe
fix -Dvp: set YY_DEBUG on DEBUG
fix gc-test segv: skip empty or invalid PN_VTABLE(ptr->vt)
debug print to STDERR, better gc-test check

also debug print GC marked objects in the GC areas
ignore gtags (global) dbs
configure for non-GNU make
travis: add DEBUG=1 env to travis
config: honor existing config.inc, pass to config.h, support ASAN=1

add static potion-s target for easier debugging
compile potion also with -fpic
Merge remote-tracking branch 'origin/master' into p2

potion.opic
Fixed conflicts:
	Makefile
	core/compile.c
	core/internal.c
	core/potion.h
backport minor p2 changes to master: jit, flags init
Merge remote-tracking branch 'origin/master' into p2

Fixed conflicts:
	core/compile.c
	core/potion.h