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/manual/local.mk

@@ -1,5 +1,3 @@
-ifeq ($(doc_generate),yes)
-
 # The version of Nix used to generate the doc. Can also be
 # `$(nix_INSTALL_PATH)` or just `nix` (to grap ambient from the `PATH`),
 # if one prefers.
@@ -180,6 +178,8 @@ manual-html: $(docdir)/manual/index.html
 install: $(docdir)/manual/index.html
 
 # Generate 'nix' manpages.
+.PHONY: manpages
+manpages: $(mandir)/man1/nix3-manpages
 install: $(mandir)/man1/nix3-manpages
 man: doc/manual/generated/man1/nix3-manpages
 all: doc/manual/generated/man1/nix3-manpages
@@ -225,5 +225,3 @@ $(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/
 	@rm -rf $(DESTDIR)$(docdir)/manual
 	@mv $(DESTDIR)$(docdir)/manual.tmp/html $(DESTDIR)$(docdir)/manual
 	@rm -rf $(DESTDIR)$(docdir)/manual.tmp
-
-endif