ext(gumbo): Makefile variable override is not necessary

since 5c6f7c1 when we started passing CFLAGS, et al through
environment variables instead of through the make command line.

Although this wasn't doing any harm on gnu-based systems, `override`
is a gnu-ism and we need to support builds on bsd systems.

Related to #2298

gumbo-parser/src/Makefile

@@ -2,7 +2,7 @@
 # to enable a mini_portile2 recipe to build the gumbo parser
 .PHONY: clean
 
-override CFLAGS += -std=c99 -Wall
+CFLAGS += -std=c99 -Wall
 
 # allow the ENV var to override this
 RANLIB ?= ranlib