ZJIT: Add macOS --with-gmp build test for ZJIT test binary

This is so the build failure in <https://github.com/ruby/ruby/pull/19138>
is covered on CI.

.github/workflows/macos.yml

@@ -36,6 +36,9 @@ jobs:
           - test_task: check
             os: macos-26
             configure_args: '--with-jemalloc --with-opt-dir=$(brew --prefix jemalloc)'
+          - test_task: zjit-test-build
+            os: macos-26
+            configure_args: '--enable-zjit=dev --with-gmp'
           - test_task: test-bundler-parallel
             os: macos-26
           - test_task: test-bundled-gems

zjit/zjit.mk

@@ -58,7 +58,7 @@ ZJIT_BINDGEN_DIFF_OPTS =
 # Generate Rust bindings. See source for details.
 # Needs `./configure --enable-zjit=dev` and Clang.
 ifneq ($(strip $(CARGO)),) # if configure found Cargo
-.PHONY: zjit-bindgen zjit-bindgen-show-unused zjit-test zjit-test-update
+.PHONY: zjit-bindgen zjit-bindgen-show-unused zjit-test zjit-test-build zjit-test-update
 .PHONY: zjit-test-debug zjit-test-lldb zjit-test-gdb zjit-test-rr
 zjit-bindgen: zjit.$(OBJEXT)
 	ZJIT_SRC_ROOT_PATH='$(top_srcdir)' BINDGEN_JIT_NAME=zjit $(CARGO) run --manifest-path '$(top_srcdir)/zjit/bindgen/Cargo.toml' -- $(CFLAGS) $(XCFLAGS) $(CPPFLAGS)
@@ -90,6 +90,13 @@ zjit-test: libminiruby.a
 	fi; \
 	exit $$exit_code
 
+# Build the `make zjit-test` binary without running it.
+zjit-test-build: libminiruby.a
+	$(ECHO) 'building ZJIT test binary'
+	$(ZJIT_NEXTEST_ENV) $(CARGO) build --tests \
+		--manifest-path '$(top_srcdir)/zjit/Cargo.toml' \
+		'--features=$(ZJIT_TEST_FEATURES)'
+
 # Accept all pending snapshots (requires cargo-insta)
 # Install with: cargo install cargo-insta
 zjit-test-update: