configure: fix curl_off_t check's include order

- Prepend srcdir include path instead of append.

Prior to this change it was possible that during the check for the size
of curl_off_t the include path of a user's already installed curl could
come before the include path of the to-be-built curl, resulting in the
system.h of the former being incorrectly included for that check.

Closes https://github.com/curl/curl/pull/1870

configure.ac

@@ -3253,7 +3253,7 @@ AC_CHECK_SIZEOF(time_t)
 AC_CHECK_SIZEOF(off_t)
 
 o=$CPPFLAGS
-CPPFLAGS="$CPPFLAGS -I$srcdir/include"
+CPPFLAGS="-I$srcdir/include $CPPFLAGS"
 AC_CHECK_SIZEOF(curl_off_t, unused , [
 #include <curl/system.h>
 ])