configure.ac PkgConfig problem with alternative pkg-config programs

[raphaelcohn]

I'm using a perl-based pkg-config program (PkgConfig) so I don't have a chicken-and-egg bootstrap problem with my toolchain. In configure.ac, the pkg config check for openssl passes openssl libssl, which causes this to bomb. This is probably a bug in the underlying PkgConfig program. However, it turns out that the .pc file for openssl contains the reference to libssl (at least in libressl), so this check can be re-written as just openssl and everything's happy...