Fix inability to find support/{platform,dirpath}.h from julia_fasttls.h.

[Sacha0]

During PackageCompiler.jl system image creation under Julia 1.7, gcc complains that it cannot find support/{platform,dirpath}.h included from julia_fasttls.h (while looking in usr/include/julia). My (potentially incorrect) working understanding is that all of julia_fasttls.h and {platform,dirpath}.h end up directly in usr/include/julia --- i.e. the support/ qualification of support/{platform,dirpath}.h effectively gets flattened away when {platform,dirpath}.h are copied from src/support/{platform,dirpath}.h to usr/include/julia. This patch drops the support/ qualification in the include in julia_fasttls.h, which locally seems to make everything happy (though I imagine this may not be the best or even a generally correct approach). Best! :)

[staticfloat]

This only works because we add -I$(JULIAHOME)/src/support to our compiler invocations. I hate different build/install pathing; I'd much rather we install our headers as ${prefix}/include/support/platform.h, for instance.

That being said, I'm okay with this, how about you @vtjnash?

[Sacha0]

This only works because we add -I$(JULIAHOME)/src/support to our compiler invocations.

Indeed :).

I hate different build/install pathing; I'd much rather we install our headers as ${prefix}/include/support/platform.h, for instance.

FWIW I share your preference. Happy to go with whichever you and other folks collectively prefer :).

[vtjnash]

We probably should restructure our headers so that the public ones (this and julia.h) are the only visible ones, and the rest are tucked away somewhere, but this seems reasonable for now

[Sacha0]

Cheers, it sounds like we would all prefer to restructure the headers at some point, but this patch does the trick for now. That being the case, I'll merge this patch to get things working pending some future restructuring.

Thanks for reviewing gentlemen! :)