Accept configure --with-cafile without filename

acinclude.m4

@@ -513,20 +513,23 @@ fi
 AC_SUBST(DEF_CAFILE)
 w3m_cafile=""
 if test x"$enable_sslverify" = xyes; then
-  AC_MSG_CHECKING(for CA bundle location)
+  AC_MSG_CHECKING(for CA file location)
   AC_ARG_WITH(cafile,
-    [   --with-cafile=CAFILE		CA file to verify SSL certificate],
-    [w3m_cafile="$with_cafile"],[
-      for f in /etc/ssl/certs/ca-certificates.crt \
-        /etc/pki/tls/certs/ca-bundle.crt \
-        /etc/ssl/ca-bundle.pem \
-        /usr/ssl/certs/ca-bundle.crt \
-        /usr/local/share/certs/ca-root-nss.crt \
-        /etc/ssl/cert.pem \
-        /etc/certs/ca-certificates.crt; do
-        if test -f "$f" -a -r "$f"; then w3m_cafile="$f"; break; fi
-      done
-    ])
+  [   --with-cafile[[=CAFILE]]	CA file to verify SSL certificate
+				default: autodetected],,[with_cafile=yes])
+  if test x"$with_cafile" = xyes; then
+    for f in /etc/ssl/certs/ca-certificates.crt \
+      /etc/pki/tls/certs/ca-bundle.crt \
+      /etc/ssl/ca-bundle.pem \
+      /usr/ssl/certs/ca-bundle.crt \
+      /usr/local/share/certs/ca-root-nss.crt \
+      /etc/ssl/cert.pem \
+      /etc/certs/ca-certificates.crt; do
+      if test -f "$f" -a -r "$f"; then w3m_cafile="$f"; break; fi
+    done
+  elif test x"$with_cafile" != xno; then
+    w3m_cafile="$with_cafile"
+  fi
   AC_MSG_RESULT($w3m_cafile)
 fi
 AC_DEFINE_UNQUOTED(DEF_CAFILE, "$w3m_cafile")