[cxx_stdlib] libc++ setting not respected

[Bjarne D Mathiesen]

I've got this in macports.conf :

cxx_stdlib         libc++
buildfromsource    always
default_compilers  macports-clang-8.0 macports-clang-7.0
macports-clang-3.9 macports-clang-3.7 gcc-4.2 apple-gcc-4.2 gcc-4.0

but

 gperf @3.1+universal
 ncurses @6.1+universal
 gettext @0.19.8.1+universal

don't respect this and still uses libstdc++
which lead to them to always show up in "port rev-ugrade" trying to
re-compile which is a PITA when compiling another port

This is seen on 10.6 Snow on both i386 and x86_64

Any ideas on how to fix this ? :-)

-- 
Bjarne D Mathiesen
Korsør ; Danmark ; Europa
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denne besked er skrevet i et (næsten) M$-frit miljø
MacOS X 10.13.6 High Sierra ; 2.8GHz Intel Core i7 ; 16GB 1067MHz DDR3