-fPIC is required to build shared objects

[yurivict]

Link fails on FreeBSD:

FAILED: lib/halide.so 
: && /usr/bin/c++ -fPIC -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing -fno-omit-frame-pointer -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing -fno-omit-frame-pointer  -fstack-protector-strong  -rdynamic -shared  -o lib/halide.so python_bindings/src/CMakeFiles/halide_py.dir/PyArgument.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyBoundaryConditions.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyBuffer.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyConciseCasts.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyDerivative.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyEnums.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyError.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyExpr.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyExternFuncArgument.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyFunc.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyFuncRef.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyHalide.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyIROperator.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyImageParam.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyInlineReductions.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyLambda.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyLoopLevel.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyMachineParams.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyModule.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyParam.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyPipeline.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyRDom.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyStage.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyTarget.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyTuple.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyType.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyVar.cpp.o python_bindings/src/CMakeFiles/halide_py.dir/PyVarOrRVar.cpp.o  -Wl,-rpath,/usr/local/llvm90/lib:  -flto  lib/libHalide.a  /usr/local/llvm90/lib/libLLVM-9.so && cd /usr/ports/lang/halide/work/.build/python_bindings/src && /usr/bin/strip /usr/ports/lang/halide/work/.build/lib/halide.so
ld: error: relocation R_X86_64_PC32 cannot be used against symbol __stack_chk_guard; recompile with -fPIC
>>> defined in /lib/libc.so.7
>>> referenced by Argument.cpp
>>>               Argument.cpp.o:(Halide::Argument::Argument(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Halide::Argument::Kind, Halide::Type const&, int, Halide::ArgumentEstimates const&)) in archive lib/libHalide.a

ld: error: can't create dynamic relocation R_X86_64_32 against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output
>>> defined in lib/libHalide.a(Argument.cpp.o)
>>> referenced by Argument.cpp
>>>               Argument.cpp.o:(Halide::Argument::Argument(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Halide::Argument::Kind, Halide::Type const&, int, Halide::ArgumentEstimates const&)) in archive lib/libHalide.a

HALIDE_SHARED_LIBRARY was OFF

[alexreinking]

This should have been fixed by #4644 -- however, we don't support or test on FreeBSD, so I can't say for sure.