Add ./configure --disable-tests option

Building without tests is useful for bootstrapping with a smaller footprint
or running the tests in a separate derivation. Otherwise, we do compile and
run them.

This isn't fine grained as to allow picking `check` but not `installcheck`
or vice versa, but it's good enough for now.

I've tried to use Nixpkgs' `checkInputs`, but those inputs weren't discovered
properly by the configure script. We can emulate its behavior very well though.

Makefile

@@ -2,13 +2,10 @@ makefiles = \
   mk/precompiled-headers.mk \
   local.mk \
   src/libutil/local.mk \
-  src/libutil/tests/local.mk \
   src/libstore/local.mk \
-  src/libstore/tests/local.mk \
   src/libfetchers/local.mk \
   src/libmain/local.mk \
   src/libexpr/local.mk \
-  src/libexpr/tests/local.mk \
   src/libcmd/local.mk \
   src/nix/local.mk \
   src/resolve-system-dependencies/local.mk \
@@ -19,12 +16,22 @@ makefiles = \
   misc/systemd/local.mk \
   misc/launchd/local.mk \
   misc/upstart/local.mk \
-  doc/manual/local.mk \
-  tests/local.mk \
-  tests/plugins/local.mk
+  doc/manual/local.mk
 
 -include Makefile.config
 
+ifeq ($(tests), yes)
+makefiles += \
+  src/libutil/tests/local.mk \
+  src/libstore/tests/local.mk \
+  src/libexpr/tests/local.mk \
+  tests/local.mk \
+  tests/plugins/local.mk
+else
+makefiles += \
+  mk/disable-tests.mk
+endif
+
 OPTIMIZE = 1
 
 ifeq ($(OPTIMIZE), 1)