fix(c): #696 - let user-set CC/AR/LD survive the build

[PYesare]

🤔 What's changed?

The c/src/Makefile unconditionally reassigned CC, LD and AR inside an ifeq/else chain. Anything that wasn't literally clang or i686-w64-mingw32-gcc fell into the else branch, which force-set CC=gcc, LD=gcc, AR=ar using plain = assignment. Plain = assignments silently override environment-set variables (only command-line-set ones are protected in GNU Make), so setting CC via the environment e.g. CC=musl-gcc make — was discarded and replaced with gcc without any warning.

This PR:

• Switches CC and AR to ?= so they're only set if the user hasn't already provided a value, otherwise falling back to make's own sensible built-in defaults (cc, ar) for the current platform.
• Renames the ad-hoc flag variables to the standard GNU Make names: CC_FLAGS → CFLAGS, AR_FLAGS → ARFLAGS, LD_FLAGS → LDFLAGS, LD_LIBS → LDLIBS, and adds SOFLAGS for the shared-library flag.
• Drops the redundant clang branch, since its flags were identical to the default.
• Keeps the i686-w64-mingw32-gcc special-case, since mingw genuinely needs different flags (no -fPIC), a dedicated archiver, and an .exe extension.

⚡️ What's your motivation?

Fixes #696. The Makefile forced users into gcc/clang/mingw and silently discarded any other compiler set via the environment, even though make's own default CC/AR values are usually already correct for the current OS.

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

I dropped the clang branch entirely since its flags matched the default, want to confirm there wasn't a reason (beyond flags) it was kept separate from the default case. Also open to feedback on whether SOFLAGS is the right name, since it's not one of GNU Make's implicit variable names like CFLAGS/ARFLAGS/LDFLAGS.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
• Users should know about my change
• I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

---

This text was originally generated from a template, then edited by hand. You can modify the template here.

[mpkorstanje]

@rfon6ngy looks good to you?

[rfon6ngy]

@mpkorstanje yes, I confirm make cli CC=x86_64-linux-musl-gcc LDFLAGS=-static working perfectly now

thanks