CPU_ARCH not set on amd64

[Alex Dupre]

The latest update of enigmail broke builds on amd64 platform. I could
fix it in the enigmail port, but I think it's an issue in the mozilla
ports on FreeBSD. Every mozilla's configure has the following sections:

=====================================================================
# Only set CPU_ARCH if we recognize the value of OS_TEST

case "$OS_TEST" in
*86 | i86pc)
    CPU_ARCH=x86
    ;;

...

x86_64 | sparc | ppc | ia64)
    CPU_ARCH="$OS_TEST"
    ;;
esac

...

if test -n "${CPU_ARCH}" -a -n "${TARGET_COMPILER_ABI}"; then
    TARGET_XPCOM_ABI="${CPU_ARCH}-${TARGET_COMPILER_ABI}"
fi
=====================================================================

So, on amd64, the CPU_ARCH variable is not set, and so neither
TARGET_XPCOM_ABI. The new enigmail Makefile doesn't correctly check for
an empty TARGET_XPCOM_ABI variable, but probably the right thing to do
is add "amd64" to the last switch case, since "amd64" should be
recognized on FreeBSD platform. Opinions?

P.S.: I've not tested the proposed patch, I looked only at the configure
code.

-- 
Alex Dupre