cmake install installs debug and release builds into same folder with same name

**Describe the bug**
debug build and install overwrites release install or vice versa (which ever is last wins)
VS2017 results in link error LNK2038 (ITERATOR_DEBUG_LEVEL) when linking a debug against release build of abseil or vice versa

Include a clear and concise description of what the problem is, including what
you expected to happen, and what actually happened.

**Steps to reproduce the bug**

cmake .. -G "Visual Studio 15 2017"
Load absl.sln into Visual Studio
Build Debug
Build Install target (verify headers and libs get installed)
Switch to Release
Build Release
Install Release
Everything gets overwritten.

For google protofbuf the debug libarary names have a 'd' appended to them so even though they are installed to same folder they co-exist.

It's important that we are able to reproduce the problem that you are
experiencing. Please provide all code and relevant steps to reproduce the
problem, including your `BUILD`/`CMakeLists.txt` file and build commands. Links
to a GitHub branch or [godbolt.org](https://godbolt.org/) that demonstrate the
problem are also helpful.

**What version of Abseil are you using?**
Live at head as of October 14 2020

**What operating system and version are you using**
Windows 10 1903

If you are using a Linux distribution please include the name and version of the
distribution as well.

**What compiler and version are you using?**
VS2017 15.9.24
Please include the output of `gcc -v` or `clang -v`, or the equivalent for your
compiler.

**What build system are you using?**
CMake
Please include the output of `bazel --version` or `cmake --version`, or the
equivalent for your build system.

**Additional context**

Add any other context about the problem here.