Revert "build: move HAVE_SANDBOX_CHECK out of MANFLAGS"

This reverts commit 5c6fa6a.

The commit in question causes `HAVE_SANDBOX_CHECK` to always be unset
(instead of only when `--disable-sandbox-check` is used), as its value
was being passed to the compiler through `MANFLAGS`.  Move the macro
back into `MANFLAGS` for simplicity.

Also, using `--disable-sandbox-check` breaks the tests and thus also
breaks CI (see #6619).

Relates to #6592.

config.mk.in

@@ -28,7 +28,6 @@ HAVE_CONTRIB_INSTALL=@HAVE_CONTRIB_INSTALL@
 HAVE_FATAL_WARNINGS=@HAVE_FATAL_WARNINGS@
 HAVE_GCOV=@HAVE_GCOV@
 HAVE_MAN=@HAVE_MAN@
-HAVE_SANDBOX_CHECK=@HAVE_SANDBOX_CHECK@
 
 # MANFLAGS
 HAVE_APPARMOR=@HAVE_APPARMOR@
@@ -45,6 +44,7 @@ HAVE_OUTPUT=@HAVE_OUTPUT@
 HAVE_OVERLAYFS=@HAVE_OVERLAYFS@
 HAVE_PRIVATE_HOME=@HAVE_PRIVATE_HOME@
 HAVE_PRIVATE_LIB=@HAVE_PRIVATE_LIB@
+HAVE_SANDBOX_CHECK=@HAVE_SANDBOX_CHECK@
 HAVE_SELINUX=@HAVE_SELINUX@
 HAVE_SUID=@HAVE_SUID@
 HAVE_USERNS=@HAVE_USERNS@
@@ -66,6 +66,7 @@ MANFLAGS = \
 	$(HAVE_OVERLAYFS) \
 	$(HAVE_PRIVATE_HOME) \
 	$(HAVE_PRIVATE_LIB) \
+	$(HAVE_SANDBOX_CHECK) \
 	$(HAVE_SELINUX) \
 	$(HAVE_SUID) \
 	$(HAVE_USERNS) \