Set AR, NM and RANLIB env. variables

[RJVB (René Bertin)]

I like to use link time optimisation where feasible and have learned through experience that this can cause link failures in projects that use static libraries (intermediate/private and/or public). This can be avoided by using a more homogeneous toolchain, i.e.

if {[string match *clang++-mp* ${configure.cxx}]} {
    configure.env-append \
                AR="[string map {"clang++" "llvm-ar"} ${configure.cxx}]" \
                NM="[string map {"clang++" "llvm-nm"} ${configure.cxx}]" \
                RANLIB="[string map {"clang++" "llvm-ranlib"} ${configure.cxx}]"
} elseif {[string match *clang-mp* ${configure.cc}]} {
    configure.env-append \
                AR="[string map {"clang" "llvm-ar"} ${configure.cc}]" \
                NM="[string map {"clang" "llvm-nm"} ${configure.cc}]" \
                RANLIB="[string map {"clang" "llvm-ranlib"} ${configure.cc}]"
}

The mentioned experience comes from Linux but I don't see why similar issues couldn't arise on Mac, for similar reasons (the default ar and/or ranlib not being able to create a usable static library from llvm objects). I think it shouldn't hurt to set the AR, NM and RANLIB env. variables as shown above from "base", along with CC, CXX etc. Or add configure.ar, configure.nm and configure.ranlib option variables?

Equivalent for in the cmake PortGroup (pre-configure block):

    if {[string match *clang++-mp* ${configure.cxx}]} {
        if {[string first "DCMAKE_AR=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_AR=[string map {"clang++" "llvm-ar"} ${configure.cxx}]
        }
        if {[string first "DCMAKE_NM=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_NM=[string map {"clang++" "llvm-nm"} ${configure.cxx}]
        }
        if {[string first "DCMAKE_RANLIB=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_RANLIB=[string map {"clang++" "llvm-ranlib"} ${configure.cxx}]
        }
    } elseif {[string match *clang-mp* ${configure.cc}]} {
        if {[string first "DCMAKE_AR=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_AR=[string map {"clang" "llvm-ar"} ${configure.cc}]
        }
        if {[string first "DCMAKE_NM=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_NM=[string map {"clang" "llvm-nm"} ${configure.cc}]
        }
        if {[string first "DCMAKE_RANLIB=" ${configure.args}] eq -1} {
            configure.args-append \
                                -DCMAKE_RANLIB=[string map {"clang" "llvm-ranlib"} ${configure.cc}]
        }
    }