This commit fixes the FindXCB.cmake module for the case no components
are requested. The previous version assigned `XCB_FIND_COMPONENTS`
list to an empty variable name.
I was lucky enough to catch the bug in a corner case where both
`XCB_COMPONENTS` and `XCB_FIND_COMPONENTS` were empty and `set()`
command failed, but note this awesome CMake language supports more annoying
corner cases like `XCB_COMPONENTS` empty and `XCB_FIND_COMPONENTS`
with two elements, which results in the following `set()` invocation:
``` cmake
set(${XCB_FIND_COMPONENTS[0]} ${XCB_FIND_COMPONENTS[1]})
```
So always beware of CMake secret charms...
9558c6d689