CMake: refactor "zigcpp" target logic

* Localize most of the global properties and functions, for some time
  they are only needed for "zigcpp" static library (sometimes with PUBLIC
  keyword, so that it will propagate to zig2): `CMAKE_*_OUTPUT_DIRECTORY`
  and two calls to `include_directories`. This removes useless flags when
  building other targets and cleans build log a bit.
* Remove `EXE_CXX_FLAGS` variable, instead use more appropriate specific
  properties and functions for this target. This gives better errors if
  compiler does not support some of them, and CMake also handles for us
  duplicate flags. It's also easier to read side-by-side with same
  flags from build.zig .
* Add some comments.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>

CMakeLists.txt

@@ -175,23 +175,6 @@ if(ZIG_STATIC_CURSES)
     list(APPEND LLVM_LIBRARIES "${CURSES}")
 endif()
 
-set(ZIG_CPP_LIB_DIR "${CMAKE_BINARY_DIR}/zigcpp")
-
-# Handle multi-config builds and place each into a common lib. The VS generator
-# for example will append a Debug folder by default if not explicitly specified.
-set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${ZIG_CPP_LIB_DIR})
-set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${ZIG_CPP_LIB_DIR})
-foreach(CONFIG_TYPE ${CMAKE_CONFIGURATION_TYPES})
-    string(TOUPPER ${CONFIG_TYPE} CONFIG_TYPE)
-    set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY_${CONFIG_TYPE} ${ZIG_CPP_LIB_DIR})
-    set(CMAKE_LIBRARY_OUTPUT_DIRECTORY_${CONFIG_TYPE} ${ZIG_CPP_LIB_DIR})
-    set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_${CONFIG_TYPE} ${CMAKE_BINARY_DIR})
-endforeach(CONFIG_TYPE CMAKE_CONFIGURATION_TYPES)
-
-include_directories(${LLVM_INCLUDE_DIRS})
-include_directories(${LLD_INCLUDE_DIRS})
-include_directories(${CLANG_INCLUDE_DIRS})
-
 find_package(Threads)
 
 set(ZIG_CONFIG_H_OUT "${CMAKE_BINARY_DIR}/config.h")
@@ -200,6 +183,7 @@ set(ZIG_CONFIG_ZIG_OUT "${CMAKE_BINARY_DIR}/config.zig")
 set(ZIG_WASM2C_SOURCES
     "${CMAKE_SOURCE_DIR}/stage1/wasm2c.c"
 )
+# Sync with "zig_cpp_sources" in build.zig
 set(ZIG_CPP_SOURCES
     # These are planned to stay even when we are self-hosted.
     "${CMAKE_SOURCE_DIR}/src/zig_llvm.cpp"
@@ -713,36 +697,64 @@ configure_file (
     "${ZIG_CONFIG_ZIG_OUT}"
 )
 
-include_directories(
-    ${CMAKE_SOURCE_DIR}
-    ${CMAKE_BINARY_DIR}
-    "${CMAKE_SOURCE_DIR}/src"
-)
+# zigcpp target
 
-if(MSVC)
-  set(EXE_CXX_FLAGS "/std:c++17")
-else()
-  set(EXE_CXX_FLAGS "-std=c++17 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D_GNU_SOURCE -fvisibility-inlines-hidden -fno-exceptions -fno-rtti -Wno-type-limits -Wno-missing-braces -Wno-comment")
-  if(MINGW)
-    set(EXE_CXX_FLAGS "${EXE_CXX_FLAGS} -Wno-format")
-  endif()
-endif()
+set(ZIGCPP_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/zigcpp")
 
 add_library(zigcpp STATIC ${ZIG_CPP_SOURCES})
-if(ZIG_PIE)
-    set(ZIGCPP_CXX_FLAGS "${EXE_CXX_FLAGS} -fno-stack-protector -fPIC")
-else()
-    set(ZIGCPP_CXX_FLAGS "${EXE_CXX_FLAGS} -fno-stack-protector")
-endif()
-set_target_properties(zigcpp PROPERTIES COMPILE_FLAGS ${ZIGCPP_CXX_FLAGS})
 
-target_link_libraries(zigcpp LINK_PUBLIC
+# Sync with minimum C++ standard required to build LLVM
+# and with "exe_cflags" in build.zig
+target_compile_features(zigcpp PRIVATE cxx_std_17)
+set_target_properties(zigcpp PROPERTIES POSITION_INDEPENDENT_CODE ${ZIG_PIE})
+
+if(NOT MSVC)
+  if(MINGW)
+    target_compile_options(zigcpp PRIVATE -Wno-format)
+  endif()
+  # Sync content below with "exe_cflags" in build.zig
+  target_compile_definitions(zigcpp PUBLIC
+    __STDC_CONSTANT_MACROS
+    __STDC_FORMAT_MACROS
+    __STDC_LIMIT_MACROS
+
+    _GNU_SOURCE
+  )
+  target_compile_options(zigcpp PRIVATE
+    -fno-exceptions
+    -fno-rtti
+    -fno-stack-protector
+
+    -fvisibility-inlines-hidden
+
+    -Wno-type-limits
+    -Wno-missing-braces
+    -Wno-comment
+  )
+endif()
+
+target_include_directories(zigcpp PUBLIC
+    ${CLANG_INCLUDE_DIRS}
+    ${LLVM_INCLUDE_DIRS}
+    ${LLD_INCLUDE_DIRS}
+)
+target_link_libraries(zigcpp PUBLIC
     ${CLANG_LIBRARIES}
     ${LLD_LIBRARIES}
     ${LLVM_LIBRARIES}
     ${CMAKE_THREAD_LIBS_INIT}
 )
 
+# Handle multi-config builds and place each into a common lib. The VS generator
+# for example will append a Debug folder by default if not explicitly specified.
+set_target_properties(zigcpp PROPERTIES ARCHIVE_OUTPUT_DIRECTORY ${ZIGCPP_OUTPUT_DIR})
+foreach(CONFIG_TYPE ${CMAKE_CONFIGURATION_TYPES})
+    string(TOUPPER ${CONFIG_TYPE} CONFIG_TYPE)
+    set_target_properties(zigcpp PROPERTIES ARCHIVE_OUTPUT_DIRECTORY_${CONFIG_TYPE} ${ZIGCPP_OUTPUT_DIR})
+endforeach()
+
+# end of zigcpp target
+
 string(TOLOWER "${CMAKE_HOST_SYSTEM_NAME}" ZIG_HOST_TARGET_OS)
 if(ZIG_HOST_TARGET_OS STREQUAL "darwin")
   set(ZIG_HOST_TARGET_OS "macos")