Make it easier to pass options to tinycc configure (#66)

This adds the `${extra_tinycc_config}` variable to the `configure`
command line so that it's possible to do something like:

    make linux-x86 extra_tinycc_config=--config-backtrace=no

Part of #62

build/deps.mk

@@ -24,7 +24,7 @@ src/embed-headers.c:
 	sed -i 's/unsigned int/const unsigned int/' src/embed-headers.c
 
 lib/tinycc/libtcc.a lib/tinycc/libtcc1.a:
-	cd lib/tinycc && ./configure ${tinycc_config}
+	cd lib/tinycc && ./configure ${tinycc_config} ${extra_tinycc_config}
 	${MAKE} -C lib/tinycc libtcc.a
 	${MAKE} -C lib/tinycc libtcc1.a