Split --disable-tests, fix cross builds

It might seem obnoxious to have yet more configure flags, but I found
controlling both the unit and functional tests with one flag was quite
confusing because they are so different:

- unit tests depending on building, functional tests don't (e.g. when
  we test already-built Nix)

- unit tests can be installed, functional tests cannot

- unit tests neeed extra libraries (GTest, RapidCheck), functional
  tests need extra executables (jq).

- unit tests are run by `make check`, functional tests are run by `make
  installcheck`

Really on a technical level, they seem wholly independent. Only on a
human level ("they are both are tests") do they have anything in common.

I had messed up the logic in cross builds because of this. Now I
split the flag in two (and cleaned up a few other inconsistencies), and
the logic fixed itself.

Co-Authored-By: Robert Hensing <roberth@users.noreply.github.com>

doc/internal-api/local.mk

@@ -1,19 +1,7 @@
-.PHONY: internal-api-html
-
-ifeq ($(internal_api_docs), yes)
-
 $(docdir)/internal-api/html/index.html $(docdir)/internal-api/latex: $(d)/doxygen.cfg
 	mkdir -p $(docdir)/internal-api
 	{ cat $< ; echo "OUTPUT_DIRECTORY=$(docdir)/internal-api" ; } | doxygen -
 
 # Generate the HTML API docs for Nix's unstable internal interfaces.
+.PHONY: internal-api-html
 internal-api-html: $(docdir)/internal-api/html/index.html
-
-else
-
-# Make a nicer error message
-internal-api-html:
-	@echo "Internal API docs are disabled. Configure with '--enable-internal-api-docs', or avoid calling 'make internal-api-html'."
-	@exit 1
-
-endif