Set proper DFLAGS.
Make sure C parts are built as PIC.

Index: runtime/CMakeLists.txt
--- runtime/CMakeLists.txt.orig
+++ runtime/CMakeLists.txt
@@ -35,7 +35,7 @@ set(MULTILIB              OFF                         
 set(BUILD_LTO_LIBS        OFF                                 CACHE BOOL   "Also build the runtime as LLVM bitcode libraries for LTO")
 set(INCLUDE_INSTALL_DIR   ${CMAKE_INSTALL_PREFIX}/include/d   CACHE PATH   "Path to install D modules to")
 set(BUILD_SHARED_LIBS     AUTO                                CACHE STRING "Whether to build the runtime as a shared library (ON|OFF|BOTH)")
-set(D_FLAGS               -w;-de;-preview=dip1000;-preview=dtorfields;-preview=fieldwise CACHE STRING "Runtime D compiler flags, separated by ';'")
+set(D_FLAGS               --relocation-model=pic;-w;-de;-preview=dip1000;-preview=dtorfields;-preview=fieldwise CACHE STRING "Runtime D compiler flags, separated by ';'")
 set(D_EXTRA_FLAGS         ""                                  CACHE STRING "Runtime extra D compiler flags, separated by ';'")
 set(D_FLAGS_DEBUG         -g;-link-defaultlib-debug;-d-debug  CACHE STRING "Runtime D compiler flags (debug libraries), separated by ';'")
 set(D_FLAGS_RELEASE       -O3;-release;-femit-local-var-lifetime  CACHE STRING "Runtime D compiler flags (release libraries), separated by ';'")
@@ -576,7 +576,7 @@ function(set_common_library_properties target name out
     # linker default on Ubuntu 16.10 and above. As we might be building on an
     # older system (e.g. binary packages), we need to make sure the C parts are
     # built as PIC as well.
-    if("${TARGET_SYSTEM}" MATCHES "Linux")
+    if("${TARGET_SYSTEM}" MATCHES "Linux|OpenBSD")
         set_target_properties(${target} PROPERTIES
             POSITION_INDEPENDENT_CODE ON)
     endif()