Forcing the include and library path is not good when the user
has a multiple prefix (e.g. /opt/local and /usr/local) for some reason.
This situation is typical for macOS (MacPorts and Homebrew).
By this change, user-specified ones are prefered over guessed prefixes.
This behavior is more straightforward because the user can have the control
of what header and library is to be linked.
Forcing the include and library path is not good when the user has a multiple prefix (e.g. /opt/local and /usr/local) for some reason. This situation is typical for macOS (MacPorts and Homebrew).
By this change, user-specified ones are prefered over guessed prefixes. This behavior is more straightforward because the user can have the control of what header and library is to be linked.