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.

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}]
        }
    }