BuildSystem: Enable Jansson in Solaris 11.

In Solaris, the header <jansson.h> is in /usr/include/jansson. To find
Jansson even in such a subdirectory, the tool pkg-config is queried via
AST_PKG_CONFIG_CHECK. For those platforms, which do not list Jansson via
pkg-config, the previous check remains and is executed thereafter.

Because the check for the NetBSD Editline library uses the tool pkg-config
the code of PKG_PROG_PKG_CONFIG must be used. Because that check happens
earlier than Jansson, it must be placed in front of that.

ASTERISK-27991

Change-Id: I69ea0f379f87a50049654b2487c76ee1c04fa53a