Add warning when compiling with iODBC support

When running configure, libiodbc2 development headers will fulfill the
requirement for ODBC development headers, but will not function
properly. This adds a warning when libiodbc2 development headers are
detected instead of unixodbc development headers.

(closes issue ASTERISK-22459)
Reported by: Patrick Maille
Tested by: Walter Doekes
Patches:
    issueA22459_warn_when_using_iodbc.patch uploaded by Walter Doekes (License 5674)
........

Merged revisions 400767 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@400768 f38db490-d61c-443f-a65b-d21fe96a405b