swift234

[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.