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>

Makefile.config.in

@@ -9,8 +9,11 @@ CXXFLAGS = @CXXFLAGS@
 CXXLTO = @CXXLTO@
 EDITLINE_LIBS = @EDITLINE_LIBS@
 ENABLE_BUILD = @ENABLE_BUILD@
+ENABLE_DOC_GEN = @ENABLE_DOC_GEN@
+ENABLE_FUNCTIONAL_TESTS = @ENABLE_FUNCTIONAL_TESTS@
+ENABLE_INTERNAL_API_DOCS = @ENABLE_INTERNAL_API_DOCS@
 ENABLE_S3 = @ENABLE_S3@
-ENABLE_TESTS = @ENABLE_TESTS@
+ENABLE_UNIT_TESTS = @ENABLE_UNIT_TESTS@
 GTEST_LIBS = @GTEST_LIBS@
 HAVE_LIBCPUID = @HAVE_LIBCPUID@
 HAVE_SECCOMP = @HAVE_SECCOMP@
@@ -36,12 +39,10 @@ checkbindir = @checkbindir@
 checklibdir = @checklibdir@
 datadir = @datadir@
 datarootdir = @datarootdir@
-doc_generate = @doc_generate@
 docdir = @docdir@
 embedded_sandbox_shell = @embedded_sandbox_shell@
 exec_prefix = @exec_prefix@
 includedir = @includedir@
-internal_api_docs = @internal_api_docs@
 libdir = @libdir@
 libexecdir = @libexecdir@
 localstatedir = @localstatedir@