Detect GCC vs Clang for compiler-specific warning flags

GCC-only flags (-Wno-format-truncation, -Wno-unused-result) are now
conditionally added via __GNUC__ / __clang__ preprocessor detection.
Fixes Clang rejecting unknown GCC flags with -Werror on CI runners.

Author: DeusData

Makefile.cbm

@@ -33,9 +33,17 @@ LIBGIT2_FLAGS =
 LIBGIT2_LIBS =
 endif
 
+# GCC-only warning suppressions (Clang rejects unknown -Wno-* with -Werror).
+# Detect GCC by checking for __GNUC__ without __clang__ — handles all versions.
+IS_GCC := $(shell echo | $(CC) -dM -E - 2>/dev/null | grep -q '__GNUC__' && ! echo | $(CC) -dM -E - 2>/dev/null | grep -q '__clang__' && echo yes || echo no)
+GCC_ONLY_FLAGS :=
+ifeq ($(IS_GCC),yes)
+GCC_ONLY_FLAGS := -Wno-format-truncation -Wno-unused-result
+endif
+
 CFLAGS_COMMON = -std=c11 -D_DEFAULT_SOURCE -Wall -Wextra -Werror \
                 -Wno-unused-parameter -Wno-sign-compare \
-                -Wno-format-truncation -Wno-unused-result \
+                $(GCC_ONLY_FLAGS) \
                 -Isrc -Ivendored -Ivendored/sqlite3 \
                 -Ivendored/mimalloc/include \
                 -I$(CBM_DIR) -I$(TS_INCLUDE) \