Define _LIBCPP_REMOVE_TRANSITIVE_INCLUDES (#1987)

Defining `_LIBCPP_REMOVE_TRANSITIVE_INCLUDES` stops libc++'s headers
from including other libc++ headers except as defined in the language
standard. This is a good idea because it will catch situations where
a header isn't explicitly included, which will cause build errors
with other standard libs like GCC's and MSVC's.

CMakeLists.txt

@@ -117,6 +117,8 @@ endif()
 add_definitions(
     -DCMAKE                  # Let the source know this is a CMAKE build
     -D__STDC_FORMAT_MACROS   # Enables printf format macros for variable sized types (e.g. size_t)
+    -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES # Stop libc++ headers from including extra headers
+
 )
 
 if(BUILD_ENTERPRISE)

Xcode/xcconfigs/Project.xcconfig

@@ -48,6 +48,13 @@ PRODUCT_NAME                                       = $(TARGET_NAME)
 
 ALWAYS_SEARCH_USER_PATHS                           = NO
 
+// defining `_LIBCPP_REMOVE_TRANSITIVE_INCLUDES` stops libc++'s headers from including other
+// libc++ headers except as defined in the language standard. This is a good idea because it will
+// catch situations where a header isn't explicitly included, which will cause build errors with
+// other standard libs like GCC's and MSVC's.
+GCC_PREPROCESSOR_DEFINITIONS = $(GCC_PREPROCESSOR_DEFINITIONS) _LIBCPP_REMOVE_TRANSITIVE_INCLUDES
+
+
 CLANG_ADDRESS_SANITIZER_CONTAINER_OVERFLOW         = YES    // range-check C++ STL containers
 CLANG_UNDEFINED_BEHAVIOR_SANITIZER_INTEGER         = NO     // don't flag int over/underflows