configure: fix curl_off_t check's include order #1870
Prior to this change it was possible that during the check for the size
This is a problem I had in original mingw, this is how I'm building:
That configure results in this test for sizeof curl_off_t, where -I./Include is last in CPPFLAGS:
Because -Ic:/mingw/msys/1.0/local/include comes before -I./Include an older curl/system.h that is already installed comes first, screwing up the conftest.
- 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 #1870