soap api: make sure we have a compatible soap extension installed

Even though the soap extension is installed, versions of PHP older than
5.2.2 do not support all the features we need. To guard against
unexpected errors, we now enable the soap extension if the
SOAP_USE_XSI_ARRAY_TYPE constant is defined. This constant is used at
runtime by the SOAP extension and is a good indicator of a recent enough
soap extension.

Fixes 0016252: API SOAP provides no answer after MantisBT upgrade