build system: make -static-libgcc selectable in config

OpenWrt wants this off.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Config.in b/Config.in
index ac5b13b..a98a8b1 100644
--- a/Config.in
+++ b/Config.in
@@ -529,6 +529,14 @@
 	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
diff --git a/Makefile.flags b/Makefile.flags
index cff935f..6ff4d72 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -58,7 +58,8 @@
 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