Make the BUILD_INTERFACE of export()ed targets work.

The existing BUILD_INTERFACE code is executed at generate time, which
is too late for export().
Export the INTERFACE_PIC property.
Test evaluation target via export for generator expressions
Merge topic 'test-export-iface-genex' into next

900dd0f Test evaluation target via export for generator expressions
c96688b Export the INTERFACE_PIC property.
2047575 Make the BUILD_INTERFACE of export()ed targets work.