CMake: Warn Unknown Build Type (#2088)

Warn if a build type passed by a user is not known in project build.

```
CMake Warning at cmake/WarpXFunctions.cmake:76 (message):
  CMAKE_BUILD_TYPE 'adsads' is not one of
  Release;Debug;MinSizeRel;RelWithDebInfo.  Is this a typo?
Call Stack (most recent call first):
  CMakeLists.txt:94 (set_default_build_type)
```

Summary:
```
  Build type: adsads (unknown type, check warning)
```

This helps if one accidentially writes `debug` instead of `Debug`,
for instance. The result of a custom type that is undefined are
no compiler flags at all...

0 files changed, 0 insertions, 0 deletions