RigCtldGUI: Doesn't use MacPorts LDFLAGS

[ryandesign (Ryan Carsten Schmidt)]

RigCtldGUI fails to build on 10.6 because although it includes the legacysupport portgroup it evidently doesn't use MacPorts LDFLAGS so it still fails to link with the legacysupport library which would have provided getline:

​https://build.macports.org/builders/ports-10.6_x86_64-builder/builds/33613/steps/install-port/logs/stdio

/opt/local/bin/clang++-mp-9.0 -o RigCtl Rig.o Hamlib.o Winkey.o rigctl_parse.o dumpcaps.o sprintflst.o fmemopen.o -lfltk -lportaudio -lhamlib -lpthread
Undefined symbols for architecture x86_64:
  "_getline", referenced from:
      get_token(__sFILE*, char*, int, char*, int) in Rig.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[ryandesign (Ryan Carsten Schmidt)]

Note also that this is using clang++ to link but isn't using MacPorts CXXFLAGS to do so (note the absence of the -stdlib flag) so that flag should be added to LDFLAGS manually when the C++ compiler is clang:

if {[string match *clang* ${configure.cxx}]} {
    configure.ldflags-append -stdlib=${configure.cxx_stdlib}
}

[Davide Gerhard <ra1nb0w@…>]

In b12e72b56db7944b22c05066237da5a62429ee6e/macports-ports (master):

RigCtldGUI: fix LDFLAGS usage

thanks ryan!

Closes: #61122