• Fix build with clang and remove USE_GCC
• Link dynamically with libavcall, libpcre and libgmp and fix build on amd64 (and optimistically sparc64 too)
• Remove the devel/libgetline dependency and don't link with libgetline: While libgetline provides getline() it has an entirely different signature than getline(3). This make the fgetline builtin function unusable and causes e.g. the cat.scm example to segfault.

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D11352