Changing things like the location of pg_config and other macros,
additional cmake flags, etc require modifying bootstrap file. This
PR instead passes any flags given to bootstrap onto cmake itself.
Users can now either force the removal of an existing BUILD_DIR
by passing BUILD_FORCE_REMOVE to ./bootstrap, or they will be
prompted to remove the dir if it already exists. Additionally, the
bootstrap script will now exit if there is an error in mkdir or
cmake.
Moving the build system to CMake allows easy cross-platform
compiles, dependency checks, and more. In particular, CMake
allows us to easily build on Windows, and Visual Studio now
has native CMake support.
