sashidown: keep -Iinclude when parent make overrides CFLAGS

CI passes CFLAGS=-Werror on the command line; that wiped the library's
CFLAGS ?= ... -Iinclude -Isrc, so fsm.h could not find sashidown.h.
Always append the include paths via override CPPFLAGS.

vendor/sashidown/Makefile

@@ -1,5 +1,8 @@
 CC ?= gcc
-CFLAGS ?= -std=c11 -Wall -Wextra -O2 -Iinclude -Isrc
+CFLAGS ?= -std=c11 -Wall -Wextra -O2
+# Keep include paths when a parent make passes CFLAGS=... on the command
+# line (CI uses -Werror that way); plain CFLAGS ?= would be overridden.
+override CPPFLAGS += -Iinclude -Isrc
 SRCS = src/fsm.c src/table_stats.c src/line_index.c src/block_scan.c \
 	src/ansi_style.c src/render.c src/render_ansi.c src/render_json.c \
 	src/sashidown.c src/compat_cmark.c src/stream.c
@@ -11,6 +14,6 @@ all: $(LIB)
 $(LIB): $(OBJS)
 	$(AR) rcs $@ $(OBJS)
 src/%.o: src/%.c
-	$(CC) $(CFLAGS) -c -o $@ $<
+	$(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<
 clean:
 	rm -f $(OBJS) $(LIB)