Compilation and the software are working fine with the default clang (cc) compiler. However, in its current version, clang does not support OpenMP, the language extension / library that we use for parallelism, which means Siril will probably be slower, for stacking in particular. Recently, clang 3.8 appeared in FreeBSD 10 with OpenMP support (clang38 in pkg), and Siril can be compiled with it using the following configure command:
C=clang38 CXX=clang38 LD=clang++38 LDFLAGS='-L/usr/local/llvm38/lib' ./autogen.sh