Build with GMime version >= 2.5.7

	* configure.in: check GMime version.
	* libbalsa/gmime-application-pkcs7.c
	(g_mime_application_pkcs7_sign), (g_mime_application_pkcs7_verify),
	(g_mime_application_pkcs7_encrypt),
	(g_mime_application_pkcs7_decrypt): build with GMime version >=
	2.5.7.
	* libbalsa/gmime-application-pkcs7.h: ditto.
	* libbalsa/gmime-gpgme-context.c (g_mime_gpgme_context_get_type),
	(g_mime_gpgme_context_class_init), (g_mime_gpgme_context_finalize),
	(g_mime_gpgme_digest_id): ditto.
	* libbalsa/gmime-gpgme-context.h: ditto.
	* libbalsa/gmime-part-rfc2440.c (g_mime_part_rfc2440_sign_encrypt),
	(g_mime_part_rfc2440_verify), (g_mime_part_rfc2440_decrypt):
	* libbalsa/gmime-part-rfc2440.h: ditto.
	* libbalsa/rfc3156.c (password_request_func),
	(libbalsa_sign_mime_object), (libbalsa_encrypt_mime_object),
	(libbalsa_body_check_signature), (libbalsa_body_decrypt): ditto.