Add workaround for building executables with older CMake versions.

Some time between CMake 3.16 and 3.17 the default behaviour seems to have changed as follows:
For a project which will build an executable, e.g.:
```
add_executable(exe_target
    main.swift)
target_link_libraries(exe_target PUBLIC
    libA
    libB)
```
In 3.17, the invocation command to create the `exe_target` executable includes `-Xlinker -rpath -Xlinkier <libA_path/lib>` and so forth for the linked libraries and their dependencies.
In 3.16, the linker is not passed the rpath flags, so the executable ends up without the `LC_RPATH` required for it to run.
This affects executable targets only, libraries do still have rpaths propagated correctly.

Author: artemcm

Sources/swift-driver/CMakeLists.txt

@@ -12,4 +12,18 @@ target_link_libraries(swift-driver PUBLIC
   SwiftDriver
   SwiftDriverExecution)
 
+# This is a fairly egregious workaround for the fact that in versions < 3.17,
+# executables do not get `-rpath` linker arguments for their linked library
+# dependencies (direct and transitive)
+if(CMAKE_VERSION VERSION_LESS 3.17)
+  get_target_property(TSC_UTIL_LIB TSCUtility LOCATION)
+  get_filename_component(TSC_LIB_DIR ${TSC_UTIL_LIB} DIRECTORY)
 
+  get_target_property(LLBUILD_LIB llbuildSwift LOCATION)
+  get_filename_component(LLBUILD_LIB_DIR ${LLBUILD_LIB} DIRECTORY)
+
+  get_target_property(ARGPARSE_LIB ArgumentParser LOCATION)
+  get_filename_component(ARGPARSE_LIB_DIR ${ARGPARSE_LIB} DIRECTORY)
+
+  target_link_options(swift-driver PUBLIC "LINKER:-rpath,${CMAKE_LIBRARY_OUTPUT_DIRECTORY},-rpath,${TSC_LIB_DIR},-rpath,${LLBUILD_LIB_DIR},-rpath,${ARGPARSE_LIB_DIR}")
+endif()

Sources/swift-help/CMakeLists.txt

@@ -13,3 +13,15 @@ target_link_libraries(swift-help PUBLIC
   ArgumentParser
   TSCBasic)
 
+# This is a fairly egregious workaround for the fact that in versions < 3.17,
+# executables do not get `-rpath` linker arguments for their linked library
+# dependencies (direct and transitive)
+if(CMAKE_VERSION VERSION_LESS 3.17)
+  get_target_property(TSC_UTIL_LIB TSCUtility LOCATION)
+  get_filename_component(TSC_LIB_DIR ${TSC_UTIL_LIB} DIRECTORY)
+  
+  get_target_property(ARGPARSE_LIB ArgumentParser LOCATION)
+  get_filename_component(ARGPARSE_LIB_DIR ${ARGPARSE_LIB} DIRECTORY)
+  
+  target_link_options(swift-help PUBLIC "LINKER:-rpath,${CMAKE_LIBRARY_OUTPUT_DIRECTORY},-rpath,${TSC_LIB_DIR},-rpath,${ARGPARSE_LIB_DIR}")
+endif()