CMake: Build zigcpp as a shared library when LLVM/Clang are shared libraries

This change intends to avoid the "Static Initialization Order Fiasco"

In general static initialization order is not guaranteed by C/C++, but
as @kubkon helpfully explained to me today (thanks!), there are some
guarantees provided by the linker (either static or dynamic) w.r.t.
initialization order that LLVM/Clang may depend upon.

These guarantees are apparently preserved when one shared library
resolves static symbols in another, but they break down when resolving
static symbols in a shared library from the main application. The concern
in this case is that the static initializers from the library may be
executed after those of the main executable.

In order to restore this ordering guarantee, we have to ensure any code
that references a static variable of a dynamically-loaded library is not
statically linked to the main application. This means that `zigcpp`
needs to be compiled as a shared library, and linked dynamically.

Author: topolarity

CMakeLists.txt

@@ -899,7 +899,12 @@ if(ZIG_TEST_COVERAGE)
     set(EXE_LDFLAGS "${EXE_LDFLAGS} -fprofile-arcs -ftest-coverage")
 endif()
 
-add_library(zigcpp STATIC ${ZIG_CPP_SOURCES})
+if (LLVM_LINK_MODE STREQUAL "shared")
+    add_library(zigcpp SHARED ${ZIG_CPP_SOURCES})
+else()
+    add_library(zigcpp STATIC ${ZIG_CPP_SOURCES})
+endif()
+
 set_target_properties(zigcpp PROPERTIES
     COMPILE_FLAGS ${EXE_CFLAGS}
 )
@@ -1005,6 +1010,17 @@ else()
   )
 endif()
 
+if(LLVM_LINK_MODE STREQUAL "shared")
+  if(APPLE)
+    set(RPATH_BASE @loader_path)
+  else()
+    set(RPATH_BASE $ORIGIN)
+  endif()
+
+  set(CMAKE_MACOSX_RPATH 1)
+  set(CMAKE_INSTALL_RPATH "${RPATH_BASE}/../lib")
+endif()
+
 # cmake won't let us configure an executable without C sources.
 add_executable(zig "${CMAKE_SOURCE_DIR}/src/stage1/empty.cpp" "${ZIG1_OBJECT}")
 
@@ -1019,7 +1035,14 @@ elseif(MINGW)
   target_link_libraries(zig ntdll)
 endif()
 
-install(TARGETS zig DESTINATION bin)
+if(LLVM_LINK_MODE STREQUAL "shared")
+  install(TARGETS zig zigcpp
+      RUNTIME DESTINATION bin
+      LIBRARY DESTINATION lib)
+else()
+  install(TARGETS zig DESTINATION bin)
+endif()
+
 
 set(ZIG_SKIP_INSTALL_LIB_FILES off CACHE BOOL
     "Disable copying lib/ files to install prefix during the build phase")