php: Fix native build due mistakenly linking against host's iconv

The configure script has mistakenly choose to enable iconv support,
due host to provide it, adding '/usr/lib' to the linking flags and
failing as:

,----[ Linking error while testing 'fork' support ]
| conftest.c:268: warning: conflicting types for built-in function 'fork'
| .../build/tmp/sysroots/x86_64-linux/usr/lib/libxml2.so:
|     undefined reference to `gzopen64@ZLIB_1.2.3.3'
| collect2: ld returned 1 exit status
`----

This patch disables iconv support for native builds fixing the error.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>