In a construct such as:

```cmake
if($ENV{FOO} MATCHES bar)
...
endif()
```

if the environment variable is not set, the if statement becomes a
syntax error because there is nothing being compared.

Fix this by quoting the environment variable like so:

```cmake
if("$ENV{FOO}" MATCHES bar)
...
endif()
```

Signed-off-by: Rafael Kitover <rkitover@gmail.com>