Remove WIN32 and APPLE references from Emscripten.cmake. NFC (#25855)

Since we are the toolchain file we decide what to set and no other
toolchain file should be setting these. We have tests that confirm that
these are indeed not set.

Also move the setting if `CMAKE_DL_LIBS` which it is not related to
`CMakeBackwardCompatibilityC.cmake` (which also had a typo in the name).

Author: sbc100

cmake/Modules/Platform/Emscripten.cmake

@@ -34,22 +34,10 @@ set(CMAKE_SYSTEM_PROCESSOR ${EMSCRIPTEN_SYSTEM_PROCESSOR})
 # This feature is activated if a shared library project has the property
 # SOVERSION defined.
 set(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG "-Wl,-soname,")
+set(CMAKE_DL_LIBS "")
 
-# In CMake, CMAKE_HOST_WIN32 is set when we are cross-compiling from Win32 to
-# Emscripten:
-# http://www.cmake.org/cmake/help/v2.8.12/cmake.html#variable:CMAKE_HOST_WIN32
-# The variable WIN32 is set only when the target arch that will run the code
-# will be WIN32, so unset WIN32 when cross-compiling.
-set(WIN32)
-
-# The same logic as above applies for APPLE and CMAKE_HOST_APPLE, so unset
-# APPLE.
-set(APPLE)
-
-# And for UNIX and CMAKE_HOST_UNIX. However, Emscripten is often able to mimic
-# being a Linux/Unix system, in which case a lot of existing CMakeLists.txt
-# files can be configured for Emscripten while assuming UNIX build, so this is
-# left enabled.
+# Emscripten has good enough Linux/Unix emualtion that it makes sense to set
+# UNIX by defaut.
 set(UNIX 1)
 
 # Do a no-op access on the CMAKE_TOOLCHAIN_FILE variable so that CMake will not
@@ -298,7 +286,7 @@ set(CMAKE_LINK_LIBRARY_USING_WHOLE_ARCHIVE_SUPPORTED True)
 # detect when building using Emscripten.
 set(EMSCRIPTEN 1 CACHE INTERNAL "If true, we are targeting Emscripten output.")
 
-# Hardwire support for cmake-2.8/Modules/CMakeBackwardsCompatibilityC.cmake
+# Hardwire support for CMake/Modules/CMakeBackwardCompatibilityC.cmake
 # without having CMake to try complex things to autodetect these:
 set(CMAKE_SKIP_COMPATIBILITY_TESTS 1)
 set(CMAKE_SIZEOF_CHAR 1)
@@ -317,7 +305,6 @@ set(CMAKE_HAVE_SYS_PRCTL_H 1)
 set(CMAKE_WORDS_BIGENDIAN 0)
 set(CMAKE_C_BYTE_ORDER "LITTLE_ENDIAN")
 set(CMAKE_CXX_BYTE_ORDER "LITTLE_ENDIAN")
-set(CMAKE_DL_LIBS)
 
 function(em_validate_asmjs_after_build target)
   message(WARNING "em_validate_asmjs_after_build no longer exists")