build: clearer diagnostic if "make" is invoked before "./configure"

build: clearer diagnostic if "make" is invoked before "./configure"

Since a GNUmakefile wrapper has been introduced in our build
system, a "make" invocation issued before a "./configure" one
has caused GNU make to die with the cryptic message:

  GNUmakefile:18: Makefile: No such file or directory
  GNUmakefile:19: /syntax-checks.mk: No such file or directory
  make: *** No rule to make target `/syntax-checks.mk'.  Stop.

* GNUmakefile: Detect when we are being run from an unconfigured
tree, and give helpful diagnostic.