cmake: set RELEASE variable to generate install target

[jwt27]

Summary

Build "Set RELEASE variable in root CMake script based on CMAKE_BUILD_TYPE, to generate install targets"

Purpose of change

The CMake scripts in the src and data subdirectories generate install targets when a RELEASE variable is set, but the root script does not define it. Perhaps users are expected to pass this to CMake manually (this is what I've been doing), but to my knowledge this is not documented anywhere.

Describe the solution

Set a RELEASE variable based on CMAKE_BUILD_TYPE in the root script.

Describe alternatives you've considered

This variable could be removed entirely, and the sub-CMakeLists could check CMAKE_BUILD_TYPE directly. I think having a separate variable is more convenient and readable.

Testing

Ran CMake with -DCMAKE_BUILD_TYPE=Release, the generated Makefile now has a working install target.

Additional context

N/A

[jbytheway]

Is there any reason not to always generate the install target? People might plausibly want to install the debug version too.