OK Robert, you can test this for me.  It fixes the mcpu flag problem, 
and copies in your CFLAGS as it should.
WARNING: this is an untested ebuild (well, it built on my iMac OK, but 
so did the original).
Cut and paste below this line if the attachment doesn't get through:
---------
# Copyright 1999-2002 Gentoo Technologies, Inc.
# Distributed under the terms of the GNU General Public License, v2 or later
# Author Achim Gottinger <achim@g.o>
# mpg123-0.59r-r1.ebuild,v 1.3 2002/05/09 23:48:34 blauwers Exp

S=${WORKDIR}/${P}

DESCRIPTION="Real Time mp3 player"
SRC_URI="http://www.mpg123.de/mpg123/${P}.tar.gz"
HOMEPAGE="http://www.mpg123.de/"

DEPEND="virtual/glibc"

src_unpack () {
    unpack ${A}
    cd ${S}
    patch -p1 < ${FILESDIR}/${P}-sparc.diff
    cp Makefile Makefile.orig
    sed -e "s:-O2 -m486:${CFLAGS}:" \
        -e "s:-O2 -mcpu=ppc:$CFLAGS:g" Makefile.orig > Makefile
}

src_compile() {
    local MAKEOPT=""
    local MAKESTYLE=""

    SYSTEM_ARCH=`echo $ARCH |\
        sed -e s/[i]*.86/i386/ \
            -e s/sun.*/sparc/ \
            -e s/arm.*/arm/ \
            -e s/sa110/arm/`

    if [ -z "$SYSTEM_ARCH" ]
    then
        SYSTEM_ARCH=`uname -m |\
        sed -e s/[i]*.86/i386/ -e s/arm.*/arm/ -e s/sa110/arm/`
    fi

    case $SYSTEM_ARCH in
      ppc)
       MAKESTYLE="-ppc";;
      i386)
       MAKESTYLE="-i486";;
      sparc64)
       MAKESTYLE="-sparc";;
      sparc)
       MAKESTYLE="-sparc";;
      arm)
       ;;
    esac

    make linux${MAKESTYLE}${MAKEOPT} || die
}

src_install () {
    into /usr
    dobin mpg123
    doman mpg123.1
    dodoc BENCHMARKING BUGS CHANGES COPYING JUKEBOX README* TODO
}