Code cleanup on CMake/Emscripten toolchain file.

Remove hardcoding that we don't need.

CMake/Modules/Urho3D-CMake-common.cmake

@@ -798,7 +798,7 @@ macro (setup_emscripten_linker_flags LINKER_FLAGS)
     else ()
         set (MEMORY_LINKER_FLAGS "-s TOTAL_MEMORY=${EMSCRIPTEN_TOTAL_MEMORY}")
     endif ()
-    set (${LINKER_FLAGS} "${${LINKER_FLAGS}} ${MEMORY_LINKER_FLAGS} -s USE_SDL=2 -s ERROR_ON_UNDEFINED_SYMBOLS=1")    # Urho3D uses SDL2 so set it here instead of in the toolchain which potentially could be reused in other projects not using SDL2
+    set (${LINKER_FLAGS} "${${LINKER_FLAGS}} ${MEMORY_LINKER_FLAGS} -s USE_SDL=2")    # Urho3D uses SDL2 so set it here instead of in the toolchain which potentially could be reused in other projects not using SDL2
     set (${LINKER_FLAGS}_RELEASE "-s AGGRESSIVE_VARIABLE_ELIMINATION=1")     # Remove variables to make the -O3 regalloc easier
     if (NOT DEFINED ENV{CI})
         set (${LINKER_FLAGS}_DEBUG -g4)     # Preserve LLVM debug information, show line number debug comments, and generate source maps

CMake/Toolchains/emscripten.toolchain.cmake

@@ -71,36 +71,17 @@ set (CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set (CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set (CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 
-# Don't do compiler autodetection, since we are cross-compiling.
-include (CMakeForceCompiler)
-CMAKE_FORCE_C_COMPILER ("${CMAKE_C_COMPILER}" Clang)
-CMAKE_FORCE_CXX_COMPILER ("${CMAKE_CXX_COMPILER}" Clang)
+# Since currently CMake does not able to identify Emscripten cross compiler, so set the compiler identification explicitly
+# However, don't force it so the rest of the compiler checks are still being performed
+set (CMAKE_C_COMPILER_ID_RUN TRUE)
+set (CMAKE_C_COMPILER_ID Clang)
+set (CMAKE_CXX_COMPILER_ID_RUN TRUE)
+set (CMAKE_CXX_COMPILER_ID Clang)
 
-# Hardwire support for cmake-2.8/Modules/CMakeBackwardsCompatibilityC.cmake without having CMake to try complex things
-# to autodetect these:
-set (CMAKE_SKIP_COMPATIBILITY_TESTS 1)
-set (CMAKE_SIZEOF_CHAR 1)
-set (CMAKE_SIZEOF_UNSIGNED_SHORT 2)
-set (CMAKE_SIZEOF_SHORT 2)
-set (CMAKE_SIZEOF_INT 4)
-set (CMAKE_SIZEOF_UNSIGNED_LONG 4)
-set (CMAKE_SIZEOF_UNSIGNED_INT 4)
-set (CMAKE_SIZEOF_LONG 4)
-set (CMAKE_SIZEOF_VOID_P 4)
-set (CMAKE_SIZEOF_FLOAT 4)
-set (CMAKE_SIZEOF_DOUBLE 8)
-set (CMAKE_C_SIZEOF_DATA_PTR 4)
-set (CMAKE_CXX_SIZEOF_DATA_PTR 4)
-set (CMAKE_HAVE_LIMITS_H 1)
-set (CMAKE_HAVE_UNISTD_H 1)
-set (CMAKE_HAVE_PTHREAD_H 1)
-set (CMAKE_HAVE_SYS_PRCTL_H 1)
-set (CMAKE_WORDS_BIGENDIAN 0)
-set (CMAKE_DL_LIBS)
-
-# In order for check_function_exists() detection to work, we must signal it to pass an additional flag, which causes the compilation
-# to abort if linking results in any undefined symbols. The CMake detection mechanism depends on the undefined symbol error to be raised.
-set (CMAKE_REQUIRED_FLAGS "-s ERROR_ON_UNDEFINED_SYMBOLS=1")
+# Set additional linker flags to consider unresolved symbols as an error
+set (CMAKE_EXE_LINKER_FLAGS "-s ERROR_ON_UNDEFINED_SYMBOLS=1")
+# Set required compiler flags for internal CMake various check_xxx() macros which rely on the error to occur for the check to be performed correctly
+set (CMAKE_REQUIRED_FLAGS ${CMAKE_EXE_LINKER_FLAGS})
 
 # Use response files on Windows host
 if (CMAKE_HOST_WIN32)