mirror: Fix features detection for building with clang

When detecting the features on the machine, clang does not need the
flags for telling the linker what libraries to use. As a consequence, it
prints a warning to let users know that the flag is unused, and this
turns into an error due to the use of -Wall and -Werror:

  clang: error: -: 'linker' input unused [-Werror,-Wunused-command-line-argument]

To address it, we can temporarily add -Wno-unused-command-line-argument
to the $(CFLAGS) when we check the features with clang.

Fixes: #3
Fixes: 28b7ccc ("mirror: Reimplement feature detection and reallocarray()")

Reported-by: @aguaiyoung
Signed-off-by: Quentin Monnet <quentin@isovalent.com>

src/Makefile.feature

@@ -2,6 +2,11 @@
 
 pound := \#
 
+CFLAGS_BACKUP := $(CFLAGS)
+ifneq ($(LLVM),)
+  CFLAGS += -Wno-unused-command-line-argument
+endif
+
 ### feature-clang-bpf-co-re
 
 feature-clang-bpf-co-re := \
@@ -103,3 +108,5 @@ $(call feature_print_status,$(HAS_LIBBFD),libbfd)
 
 $(foreach feature,$(filter-out libbfd,$(FEATURE_DISPLAY)), \
   $(call feature_print_status,$(feature-$(feature)),$(feature)))
+
+CFLAGS := $(CFLAGS_BACKUP)