legacysupport: current support for 'macports-libcxx' broken

[mascguy (Christopher Nielsen)]

Reviewing the two ports that currently utilize macports-libcxx - cherrytree and retdec-devel - the following is necessary to enable use of it:

depends_lib-append \
    port:macports-libcxx

configure.cxxflags-append \
    -nostdinc++ \
    -I${prefix}/include/libcxx/v1

configure.ldflags-append \
    -L${prefix}/lib/libcxx

While the legacysupport PG is correctly adding the lib dependency, along with the lib path, there are two issues:

• Compilation argument -nostdinc++ isn't being added
• The header path is missing the trailing /v1

If these issues are fixed, then we may be able to simplify ports cherrytree and retdec-devel, via the legacysupport PG.