php7: try to fix compilation on LEDE buildbots

Detection of U8T_DECOMPOSE seems to be broken when cross-compiling,
so needs to be preseeded.

-snip-
checking for utf8_mime2text signature... new
checking for U8T_DECOMPOSE...
configure: error: utf8_mime2text() has new signature, but U8T_CANONICAL
 is missing. This should not happen. Check config.log for additional information.
-snap-

This requires also a patch for PHP to make the preseeding working.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

lang/php7/Makefile

@@ -473,6 +473,7 @@ CONFIGURE_VARS+= \
 	php_cv_cc_rpath="no" \
 	iconv_impl_name="gnu_libiconv" \
 	ac_cv_php_xml2_config_path="$(STAGING_DIR)/host/bin/xml2-config" \
+	ac_cv_u8t_decompose=yes \
 
 define Package/php7/conffiles
 /etc/php.ini

lang/php7/patches/1007-ext-imap-config.m4-fix-ac_cv_u8t_decompose-check.patch

@@ -0,0 +1,35 @@
+From 96465be0f4ac225cfe6f52b809c91b9496807eac Mon Sep 17 00:00:00 2001
+From: Michael Heimpold <mhei@heimpold.de>
+Date: Sun, 26 Nov 2017 18:29:42 +0100
+Subject: [PATCH] ext/imap/config.m4: fix ac_cv_u8t_decompose check
+
+Once upon the time, commit c58f63a38ae19caaab339c61486fc3bd7e5894f9
+changed the check from U8T_CANONICAL to U8T_DECOMPOSE. However,
+the autoconf cache id was not renamed.
+
+Sometimes it is desirable to preseed the autoconf variables, e.g. when
+cross-compiling to avoid the tests running on the host system. In this
+case it's confusing when the cache id does not match the variable to
+set, so let's adjust it.
+
+Signed-off-by: Michael Heimpold <mhei@heimpold.de>
+---
+ ext/imap/config.m4 | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/ext/imap/config.m4 b/ext/imap/config.m4
+index d0b86cb..badb6e2 100644
+--- a/ext/imap/config.m4
++++ b/ext/imap/config.m4
+@@ -147,7 +147,7 @@ if test "$PHP_IMAP" != "no"; then
+ 
+     old_CFLAGS=$CFLAGS
+     CFLAGS="-I$IMAP_INC_DIR"
+-    AC_CACHE_CHECK(for U8T_DECOMPOSE, ac_cv_u8t_canonical,
++    AC_CACHE_CHECK(for U8T_DECOMPOSE, ac_cv_u8t_decompose,
+       AC_TRY_COMPILE([
+ #include <c-client.h>
+       ],[
+-- 
+2.7.4
+