abiword-x11 @3.0.4: Does not use MacPorts-provided LDFLAGS so build fails

[ryandesign (Ryan Carsten Schmidt)]

abiword-x11 fails to build on OS X 10.11 and earlier:

​https://build.macports.org/builders/ports-10.9_x86_64-builder/builds/141894/steps/install-port/logs/stdio

Undefined symbols for architecture x86_64:
  "_clock_gettime", referenced from:
      GR_Caret::_blink(bool) in libxp.a(gr_Caret.o)
ld: symbol(s) not found for architecture x86_64

I added the legacysupport portgroup, but the LDFLAGS that the portgroup adds (and the other LDFLAGS that MacPorts always adds) are not being used by the port, therefore the build fails.

[ryandesign (Ryan Carsten Schmidt)]

Duplicate of #62410.