set CPATH so go finds C headers with macOS Catalina

:-(

Makefile

@@ -11,6 +11,9 @@ PRELOAD_SRC = $(PRELOAD_LISP) pre/*.go.*
 GO_TEST_MODULES = lob olu
 REGISTER_BUILTINS = fun/Register_Builtins.go
 
+# doesn't find C include files with macOS Catalina (10.15)
+CPATH = /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include
+
 # remote repositories, sorted reverse so origin is in front
 REMOTES = $$(git remote | sort -r)
 
@@ -40,6 +43,7 @@ format:
 
 # check for some kinds of errors
 checksrc:
+	export CPATH=$(CPATH)
 	go vet .
 
 # etags works with Go, too!
@@ -48,6 +52,7 @@ etags:
 
 # the actual build rule
 lingo: gen/lisp-preload_.go buildtag $(REGISTER_BUILTINS) checksrc
+	export CPATH=$(CPATH)
 	go get github.com/peterh/liner
 	go build $(BUILDFLAGS)
 	./lingo -e '(exit)'            # check preload code