[gmime] Fixed configure.ac on Linux

[Jeffrey Stedfast <fejj src gnome org>]

commit dda58179411a8e6e9af982fa80d613b5dc9a5a14
Author: Jeffrey Stedfast <fejj gnome org>
Date:   Fri Dec 9 19:53:15 2016 -0500

    Fixed configure.ac on Linux

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index d92a5d7..aac3fe2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -306,9 +306,9 @@ AS_IF([test x"$glib_native_win32" != xyes], [
 AC_SUBST(ICONV_LIBS)
 
 case $with_libiconv in
-  gnu)
+  gnu|no)
     AC_DEFINE(USE_LIBICONV_GNU, 1, [Using GNU libiconv])
-    AC_DEFINE(ICONV_CONST, const, [GNU libiconv uses const char**])
+    AC_DEFINE(ICONV_CONST, [const], [GNU libiconv uses const char**])
     ;;
   native)
     AC_DEFINE(USE_LIBICONV_NATIVE, 1, [Using a native implementation of iconv in a separate library])