[gmime] build: Use m4_ifdef for GPGME support

[Colin Walters <walters src gnome org>]

commit 4a7d6e63c912af34caaf9afb2f8155d548e6fe44
Author: Colin Walters <walters verbum org>
Date:   Mon Jan 13 19:25:38 2014 -0500

    build: Use m4_ifdef for GPGME support
    
    This fixes the build if GPGME's m4 macros are not in the buildroot.

 configure.ac |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 1f19121..9d78c7d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -498,12 +498,14 @@ if test "x$enable_cryptography" != "xyes"; then
    enable_smime="no"
 fi
 
-if test "x$enable_smime" = "xyes"; then
+m4_ifdef([AM_PATH_GPGME_PTHREAD], [
+AS_IF([test "x$enable_smime" = "xyes"], [
    AM_PATH_GPGME_PTHREAD(1.1.6, enable_smime="yes (via GpgME)", enable_smime="no")
    if test "x$enable_smime" != "xno"; then
       AC_DEFINE(ENABLE_SMIME, 1, [Define if GMime should enable experimental S/MIME support.])
    fi
-fi
+])
+])
 
 dnl ****************************
 dnl *** Enable Mono bindings ***