PHPC's m4 scripts contain some checks for Solaris, which date back to at least to f26f78e and version 1.5.0. The checks were originally lifted from libmongoc's old m4 scripts (CDRIVER-2015).

While bumping PHPC to libmongoc 1.21-dev, I realized these old lines might conflict with some newer build definitions in mongodb/mongo-c-driver@1523c42 for CDRIVER-4233. I noted that libmongoc's current cmake scripts make no mention of Solaris and kevin.albertson pointed out that Solaris support was removed some time ago in CDRIVER-2230.

AFAIK, PHPC has never tested on Solaris and we don't document support for that platform anywhere. We should remove these old checks, which will avoid any possible conflict with porting over the new cmake lines to autotools when we upgrade to libmongoc 1.21-dev.