CXX-2726 do not override CMAKE_BUILD_TYPE for multi-config generator

[kevinAlbs]

Summary

• do not override CMAKE_BUILD_TYPE for multi-config generator
• remove CMAKE_BUILD_TYPE from Windows install instructions
• specify --config in Windows install instructions

Background & Motivation

Configuring on Windows with Visual Studio results in this log output from CMake:

-- No build type selected, default is Release

The log message is printed when overriding CMAKE_BUILD_TYPE. Overriding CMAKE_BUILD_TYPE is ignored for multi-configuration generators:

For multi-configuration generators like Visual Studio, Xcode, and Ninja Multi-Config, the configuration is chosen by the user at build time and CMAKE_BUILD_TYPE is ignored.

This may mislead users to think the build is Release, when it may be Debug. This may result in confusing runtime errors if mixing Debug and Release builds in an application. This PR changes to not override CMAKE_BUILD_TYPE for multi-configuration generator.

See CXX-2726 for user reports motivating this change.

---

To configure and build on Windows, the following commands were used:

$CMAKE \
    -Thost=x64 -A x64 \
    -DCMAKE_PREFIX_PATH="C:\cygwin\home\Administrator\code\c-bootstrap\install\mongo-c-driver-1.24.2x64" \
	-DCMAKE_INSTALL_PREFIX="./.install" \
	-DBUILD_VERSION=3.9.0-pre \
        -DCMAKE_CXX_STANDARD=17 \
	-S. -B./cmake-build

# To build with default config (resulted in Debug):
$CMAKE --build cmake-build --target mongocxx_shared --parallel 16

# To build with specified config:
$CMAKE --build cmake-build --target mongocxx_shared --parallel 16 --config RelWithDebInfo