build: correct the inverted logic for the nanorc man page

All this "not is no" has me turning around in circles
at least ten times before I know which side is which.

This fixes https://savannah.gnu.org/bugs/?51061
Reported-by: Vicente Olivert Riera <vincent.riera@imgtec.com>

configure.ac

@@ -201,7 +201,6 @@ fi
 if test "x$disable_nanorc" != xyes; then
     if test "x$enable_nanorc" != xno; then
 	AC_DEFINE(ENABLE_NANORC, 1, [Define this to enable the use of .nanorc files.])
-    else
 	nanorc_support=yes
     fi
 fi