Provide error hint if TAP tests are not enabled

The error message for trying to run the TAP tests in a tree not
configured with --enable-tap-tests is quite terse, and could be
made more helpful to new developers onboarding to postgres. This
adds a small hint on how to get the tests running in such cases.

Author: Kevin Burke <kevin@burke.dev>
Discussion: https://postgr.es/m/CAKcy5ejKVYwUXguQcd6i9KHDm7cM7FzjQ+aayaPveoa_woyQpQ@mail.gmail.com

Author: danielgustafsson

src/Makefile.global.in

@@ -476,7 +476,7 @@ cd $(srcdir) && \
 endef
 
 else
-prove_installcheck = @echo "TAP tests not enabled"
+prove_installcheck = @echo "TAP tests not enabled. Try configuring with --enable-tap-tests"
 prove_check = $(prove_installcheck)
 endif