Makefile: Add ARFLAGS for overriding command line options to ar

- Makefile.base: Respect ARFLAGS when building static archives.
 - Makefile.cflags: Add default ARFLAGS.
 - Makefile.vars: Add description for ARFLAGS.

Makefile.base

@@ -49,7 +49,7 @@ $(BINDIR)$(MODULE)/:
 $(BINDIR)$(MODULE).a $(OBJ): | $(BINDIR)$(MODULE)/
 
 $(BINDIR)$(MODULE).a: $(OBJ) | ${DIRS:%=ALL--%}
-	$(AD)$(AR) -rcs $@ $?
+	$(AD)$(AR) $(ARFLAGS) $@ $?
 
 
 CXXFLAGS = $(filter-out $(CXXUWFLAGS), $(CFLAGS)) $(CXXEXFLAGS)

Makefile.cflags

@@ -50,3 +50,9 @@ endif
 
 # Forbid common symbols to prevent accidental aliasing.
 CFLAGS += -fno-common
+
+# Default ARFLAGS for platforms which do not specify it.
+# Note: make by default provides ARFLAGS=rv which we want to override
+ifeq ($(origin ARFLAGS),default)
+  ARFLAGS = rcs
+endif

Makefile.vars

@@ -26,6 +26,7 @@ export CFLAGS                # The compiler flags. Must only ever be used with `
 export CXXUWFLAGS            # (Patters of) flags in CFLAGS, that should not be passed to CXX.
 export CXXEXFLAGS            # Additional flags that should be passed to CXX.
 export AR                    # The command to create the object file archives.
+export ARFLAGS               # Command-line options to pass to AR, default `rcs`.
 export AS                    # The assembler.
 export ASFLAGS               # Flags for the assembler.
 export LINK                  # The command used to link the files. Must take the same parameters as GCC, i.e. "ld" won't work.