[cmake] Change CMake Policy CMP0054 for Cygwin (#4274)

In CMake 3.4.3, if `CMP0054` policy isn't set to any value, `cmake`
uses the OLD behavior and issues a long warning message for every
`if()` statement having a quoted variable whose value is an other
variable's name.

For example,
```
set(FOO 1)
set(VAR_A FOO)
set(VAR_B BAR)
if ("${VAR_A}" STREQUAL "FOO")
endif()
if ("${VAR_B}" STREQUAL "BAR")
endif()
```
issues `CMP0054` warning message for `"${VAR_A}"` and not for
`"${VAR_B}"`.

Cygwin `cmake` defines the variable `CMAKE_SYSTEM_NAME` as `CYGWIN`
and defines the variable `CYGWIN` as 1(TRUE).

Linux `cmake` defines the variable `CMAKE_SYSTEM_NAME` as `Linux`
and there is no variable `Linux`.

So, if we didn't set any value to `CMP0054` policy, every
`if ("${CMAKE_SYSTEM_NAME}" ...)` generated long warning message in
Cygwin, but did not generate in Linux.

Author: tinysun212

CMakeLists.txt

@@ -4,6 +4,10 @@ if(POLICY CMP0051)
   cmake_policy(SET CMP0051 NEW)
 endif()
 
+if(POLICY CMP0054)
+  cmake_policy(SET CMP0054 NEW)
+endif()
+
 # Add path for custom CMake modules.
 list(APPEND CMAKE_MODULE_PATH
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")

stdlib/public/core/CMakeLists.txt

@@ -174,7 +174,7 @@ else()
       ${ICU_UC_LIBRARY} ${ICU_I18N_LIBRARY})
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "CYGWIN")
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "CYGWIN")
   execute_process(COMMAND "cygpath" "-u" "$ENV{SYSTEMROOT}"
       OUTPUT_VARIABLE ENV_SYSTEMROOT)