base: ports cannot use SDK inside of Developer dir unless using Xcode build type

[Ionic (Mihai Moldovan)]

Currently, base only allows allows to the SDK directory of ports use the Xcode build type or explicitly state that they use Xcode or the SDK directory lies outside of the Developer directory.

This behavior, though, is too restrictive.

One example of this failing badly is py35-pyqt5:

:info:configure Error: Failed to determine the detail of your Qt installation. Try again using
:info:configure the --verbose flag to see more detail about the problem.
:info:configure Querying qmake about your Qt installation...
:info:configure Determining the details of your Qt installation...
:info:configure /opt/local/libexec/qt5/bin/qmake -o cfgtest_QtCore.mk cfgtest_QtCore.pro
:info:configure Project ERROR: Could not resolve SDK Path for 'macosx10.9'

That's likely not isolated to this port, but any port using the qmake5-1.0 PG or actually even qmake(5) directly.

Base should allow access to the SDK dir if configure.sdk_version is set, regardless of whether configure.sdkroot is the default/set to an empty value (and hence relative to the Developer dir) or not.