build system: make -static-libgcc selectable in config

OpenWrt wants this off.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Config.in

@@ -529,6 +529,14 @@ config STACK_OPTIMIZATION_386
 	do not work with it (they use SSE instructions without
 	ensuring stack alignment).
 
+config STATIC_LIBGCC
+	bool "Use -static-libgcc"
+	default y
+	help
+	This option instructs gcc to link in a static version of its
+	support library, libgcc. This means that the binary will require
+	one fewer dynamic library at run time.
+
 comment 'Installation Options ("make install" behavior)'
 
 choice

Makefile.flags

@@ -58,7 +58,8 @@ CFLAGS += $(call cc-option,-fno-builtin-strlen -fomit-frame-pointer -ffunction-s
 CFLAGS += $(call cc-option,-fno-guess-branch-probability,)
 CFLAGS += $(call cc-option,-funsigned-char,)
 
-ifneq ($(CC),clang)
+ifeq ($(CONFIG_STATIC_LIBGCC),y)
+# Disable it, for example, if you get
 # "clang-9: warning: argument unused during compilation: '-static-libgcc'"
 CFLAGS += $(call cc-option,-static-libgcc,)
 endif