CMake: Merge private dependencies into single static objects archive

When building the Testing library as a static library
(BUILD_SHARED_LIBS=FALSE), `_TestingInternals` is not included in the
`libTesting.a` archive by default. (when building as a shared library,
`libTesting.so` includes `_TestingInternals`). This causes the linker to
complain about missing symbols, so we need to manually merge the objects
from `_TestingInternals` into `libTesting.a`.

Author: kateinoigakukun

Sources/Testing/CMakeLists.txt

@@ -94,6 +94,11 @@ add_library(Testing
   Traits/Trait.swift)
 target_link_libraries(Testing PRIVATE
   _TestingInternals)
+if(NOT BUILD_SHARED_LIBS)
+  # Merge private dependencies into single static objects archive
+  set_property(TARGET Testing PROPERTY STATIC_LIBRARY_OPTIONS
+    $<TARGET_OBJECTS:_TestingInternals>)
+endif()
 add_dependencies(Testing
   TestingMacros)
 target_compile_options(Testing PRIVATE