build: refactor the rpath handling

The rpath handling uses linker flags specific to the ELF linkers.  This
is in preparation to build for Windows which does does not have the
concept of `DT_RPATH`.

Author: compnerd

CMakeLists.txt

@@ -102,10 +102,12 @@ endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL Android OR CMAKE_SYSTEM_NAME STREQUAL Linux)
   set(deployment_target -DDEPLOYMENT_TARGET_LINUX)
+  set(Foundation_rpath_flags -Xlinker;-rpath;-Xlinker;"\\\$\$ORIGIN")
 elseif(CMAKE_SYSTEM_NAME STREQUAL Darwin)
   set(deployment_target -DDEPLOYMENT_TARGET_MACOSX)
 elseif(CMAKE_SYSTEM_NAME STREQUAL FreeBSD)
   set(deployment_target -DDEPLOYMENT_TARGET_FREEBSD)
+  set(Foundation_rpath_flags -Xlinker;-rpath;-Xlinker;"\\\$\$ORIGIN")
 elseif(CMAKE_SYSTEM_NAME STREQUAL Windows)
   set(deployment_target -DDEPLOYMENT_TARGET_WINDOWS)
 endif()
@@ -287,7 +289,7 @@ add_swift_library(Foundation
                     ${libdispatch_ldflags}
                     -L${CMAKE_CURRENT_BINARY_DIR}
                     -luuid
-                    -Xlinker;-rpath;-Xlinker;"\\\$\$ORIGIN"
+                    ${Foundation_rpath_flags}
                   SWIFT_FLAGS
                     -DDEPLOYMENT_RUNTIME_SWIFT
                     ${deployment_enable_libdispatch}