qt5-build.eclass: preliminary support for Qt 5.7

qt5-build.eclass: preliminary support for Qt 5.7

This is a squashed commit of the following cherry picks from proj/qt.git:

qt5-build.eclass: update licensing options for Qt 5.7
  (cherry picked from 0b87664bd7a24f3b07117ec6cc059898b6a33aac)

qt5-build.eclass: tweak sed to apply to avx512 tests added in 5.7
  (cherry picked from 7c3bcf6032b37bcb15b8bac6dba1b151e80d613d)

qt5-build.eclass: rewrite sed so that it works on 5.6 and 5.7 too
  (cherry picked from 12f4fabfaca9bd4c0f62d8ba0b9731840eff588e)

qt5-build.eclass: tweak sed to apply to 5.7's config.tests
  (cherry picked from ab4ed663cee3126d99e6638a0607531d589194be)

qt5-build.eclass: adapt qmake installation target name for 5.7
  (cherry picked from 154f21fa9b3677d079ddf1ac5bad5cbedf792ee7)