Fix building on FreeBSD via cmake (swiftlang#925)

Currently building swift-testing via cmake (e.g. when building the
toolchain) fail if not compile as PIC. Adding "-fPIC" fixed this
problem. Moreover, we need to link against `libexecinfo` on FreeBSD
where the symbol `backtrace` lives.

### Result:

Swift testing can now successfully build via cmake thus installed to the
toolchain 🥳

Author: michael-yuji

Sources/CMakeLists.txt

@@ -66,7 +66,7 @@ if(SwiftTesting_MACRO STREQUAL "<auto>")
   if(NOT SwiftTesting_BuildMacrosAsExecutables)
     if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
       set(SwiftTesting_MACRO_PATH "${SwiftTesting_MACRO_INSTALL_PREFIX}/lib/swift/host/plugins/testing/libTestingMacros.dylib")
-    elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux")
+    elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux" or CMAKE_HOST_SYSTEM_NAME STREQUAL "FreeBSD")
       set(SwiftTesting_MACRO_PATH "${SwiftTesting_MACRO_INSTALL_PREFIX}/lib/swift/host/plugins/libTestingMacros.so")
     elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
       set(SwiftTesting_MACRO_PATH "${SwiftTesting_MACRO_INSTALL_PREFIX}/bin/TestingMacros.dll")

Sources/Testing/CMakeLists.txt

@@ -113,6 +113,9 @@ if(NOT APPLE)
   endif()
   target_link_libraries(Testing PUBLIC
     Foundation)
+  if (CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+    target_link_libraries(Testing PUBLIC execinfo)
+  endif()
 endif()
 if(NOT BUILD_SHARED_LIBS)
   # When building a static library, tell clients to autolink the internal

Sources/_TestingInternals/CMakeLists.txt

@@ -20,6 +20,9 @@ if("${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "MSVC" OR
    "${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   target_compile_options(_TestingInternals PRIVATE
     /EHa-c)
+elseif(CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+  target_compile_options(_TestingInternals PRIVATE
+    -fno-exceptions -fPIC)
 else()
   target_compile_options(_TestingInternals PRIVATE
     -fno-exceptions)