qmake5 PortGroup: do not include unneeded legacysupport

[tuffnatty]

Description

Building qt5-qttools fails (ticket, logs) on Mojave and High Sierra with a cryptic message:

Undefined symbols for architecture x86_64:
  "vtable for ApplicationEventFilter", referenced from:
      _main in main.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.

It looks like the problem goes away when I remove -isystem/opt/local/include/LegacySupport from the compiler invocation. qt5-qttools does not use legacysupport itself. legacysupport is brought in by qmake5-1.0.tcl, which does not seem to make any use of it. Removing legacysupport from qmake5-1.0.tcl fixes the build, at least on Mojave.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.14.6 18G9323 x86_64
Command Line Tools 10.3.0.0.1.1562985497

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?