Need BuildLibs_rel to support static library in release, dynamic library in debug

Description:

A common development practice is to use a library that links dynamically to the CRT in debug builds, and statically in release builds. We have Libs_rel and Libs_dbg that let us specify the different library names if we are linking without building. However, we have only one BuildLibs option.

We should let BuildLibs stay as is for common and debug builds, but if the user needs to differentiate release builds, then we should support a BuildLibs_rel option which would override BuildLibs when building a release version. For consistency, we should also support a BuildLibs_dbg option (which internally would be identical to BuildLibs)

Issue Type:

• Bug report
• Feature request
• Design change request
• Other