JIT stencil compilation does not respect CFLAGS

[zanieb]

If CFLAGS are set during configure, they are not propagated to the clang invocation to build the JIT stencils. This is problematic for downstream consumers, e.g., in python-build-standalone where we're using an isolated build system and the clang invocation fails to find required includes.

Linked PRs

• gh-134273: Propagate CFLAGS to the JIT #134274
• gh-134273: Allow setting JIT compiler flags with CFLAGS_JIT #134276
• [3.14] GH-134273: Allow setting JIT compiler flags at build time with CFLAGS_JIT (GH134276) #135792