ext(gumbo): avoid Gnu-isms in the Makefile

The wildcard and patsubst functions, as well as $^, don't work in BSD
make. Let's just inline everything to make it simple and portable.

Related to #2298

gumbo-parser/src/Makefile

@@ -7,10 +7,27 @@ CFLAGS += -std=c99 -Wall
 # allow the ENV var to override this
 RANLIB ?= ranlib
 
-gumbo_objs := $(patsubst %.c,%.o,$(wildcard *.c))
+gumbo_objs := \
+	ascii.o \
+	attribute.o \
+	char_ref.o \
+	error.o \
+	foreign_attrs.o \
+	parser.o \
+	string_buffer.o \
+	string_piece.o \
+	svg_attrs.o \
+	svg_tags.o \
+	tag.o \
+	tag_lookup.o \
+	token_buffer.o \
+	tokenizer.o \
+	utf8.o \
+	util.o \
+	vector.o
 
 libgumbo.a: $(gumbo_objs)
-	$(AR) $(ARFLAGS) $@ $^
+	$(AR) $(ARFLAGS) $@ $(gumbo_objs)
 	- ($(RANLIB) $@ || true) >/dev/null 2>&1
 
 clean: