embedding issues

[JeffBezanson]

Trying to run the embedding example I'm hitting a number of issues. Some I can fix myself (in an upcoming PR) but others I'll need help with:

• Running make gives

/home/jeff/src/julia/Make.inc:120: *** cowardly refusing to build from source directory with a shell-metacharacter in the path (got: $(abspath ..)).  Stop.

• The world age is 0, causing e.g. the jl_show call in embedding.c to fail. Adding jl_get_ptls_states()->world_age = jl_world_counter; to jl_init fixes it, but I wouldn't expect that to be necessary. Where should this initialization happen?

These I can fix unless there are objections:

• Need to add JCPPFLAGS and JCFLAGS to examples/Makefile.
• Default system image path when calling jl_init is broken.

[ararslan]

cc @tkelman and @staticfloat for point 1, @vtjnash for point 2

[vtjnash]

the jl_show call

...should be gotten rid of

we don't need yet another entry point now that we have jl_call and cfunction

[staticfloat]

@JeffBezanson see #21140 for the incredibly in-depth and elaborate workaround required to fix part one.

[JeffBezanson]

Fixed by #21299