FetchContent requires CMake 3.11 (released Mar 28, 2018).
Fixed this warning:
```
CMake Warning (dev) at /usr/share/cmake/Modules/FetchContent.cmake:1316 (message):
The DOWNLOAD_EXTRACT_TIMESTAMP option was not given and policy CMP0135 is
not set. The policy's OLD behavior will be used. When using a URL
download, the timestamps of extracted files should preferably be that of
the time of extraction, otherwise code that depends on the extracted
contents might not be rebuilt if the URL changes. The OLD behavior
preserves the timestamps from the archive instead, but this is usually not
what you want. Update your project to the NEW behavior or specify the
DOWNLOAD_EXTRACT_TIMESTAMP option with a value of true to avoid this
robustness issue.
Call Stack (most recent call first):
imgui/CMakeLists.txt:23 (FetchContent_Declare)
```
