Clarify OS X build instructions
Clarify OS X build instructions

* Polish documentation for building p4c on OS X.

* Clarify role of xcode-select.

* Add pkg-config (was needed to make GNU Automake tools work on my machine).

* Specify non-standard GMP C++ flags.
p4lang/p4c,hanw/p4c,hanw/p4c,mbudiu-vmw/p4c-clone,hanw/p4c,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,p4lang/p4c,mbudiu-vmw/p4c-clone,p4lang/p4c,p4lang/p4c,p4lang/p4c,hanw/p4c