Split regress-binaries into two targets.

Split the binaries for the unit tests out into a regress-unit-binaries
target, and add a dependency on it for only the unit tests.  This allows
us to run the integration tests only ("make t-exec") without building
the unit tests, which allows us to run a subset of the tests when
building --without-openssl without trying (and failing) to build the
unit tests.

This means there are two targets for "unit" which I *think* is valid
(it works in testing, and makedepend will generate Makefiles of this
form)a but I could be wrong.

Makefile.in

@@ -576,7 +576,9 @@ regress-binaries: regress/modpipe$(EXEEXT) \
 	regress/setuid-allowed$(EXEEXT) \
 	regress/netcat$(EXEEXT) \
 	regress/check-perm$(EXEEXT) \
-	regress/mkdtemp$(EXEEXT) \
+	regress/mkdtemp$(EXEEXT)
+
+regress-unit-binaries: \
 	regress/unittests/sshbuf/test_sshbuf$(EXEEXT) \
 	regress/unittests/sshkey/test_sshkey$(EXEEXT) \
 	regress/unittests/bitmap/test_bitmap$(EXEEXT) \
@@ -587,6 +589,8 @@ regress-binaries: regress/modpipe$(EXEEXT) \
 	regress/unittests/utf8/test_utf8$(EXEEXT) \
 	regress/misc/kexfuzz/kexfuzz$(EXEEXT)
 
+unit: regress-unit-binaries
+
 tests interop-tests t-exec unit: regress-prep regress-binaries $(TARGETS)
 	BUILDDIR=`pwd`; \
 	TEST_SSH_SCP="$${BUILDDIR}/scp"; \