set() does not evaluate conditional expressions like if() does.
BUILD_CPPDAP was being set to "${BUILD_VULKAN};AND;${ENABLE_VULKAN_DEBUGGER}", so cppdap was always required as a submodule dependency.

Bug: b/145351270
Change-Id: I86c3e43b8e22aa283bd2a5a20e0c03c01a965e8a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39310Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>