build: use a more dependable method for detecting a build from git

The Haiku recipe seems to detect a .git directory somewhere, even though
they are building from a tarball.  So, check instead for the presence of
nano's release script, which ought to be present only in a git checkout,
not in a released tarball.

configure.ac

@@ -59,7 +59,7 @@ AC_DEFINE_DIR([PKGDATADIR], [pkgdatadir], [Where data are placed to.])
 dnl Whether this is a git repository.
 
 AC_MSG_CHECKING([whether building from git])
-if test -d .git ; then
+if test -f roll-a-release.sh ; then
 	AC_MSG_RESULT([yes])
 	from_git=yes
 else