[complib:mpfr] Fix building MPFR in some weird cases

The tmul test uses a compiled-in input file in $(srcdir).
The problem is that the Makefile passes it unquoted. The C code
tries to stringify it using clever macros, which may *usually* work.

In my case the source directory was named:
.../toolchain-powerpc-e500v2-linux-gnuspe-1.0-2.fc10/.../tests
And guess what? During testing I found out the program fails because
it tries to open:
.../toolchain-powerpc-e500v2-1-gnuspe-1.0-2.fc10/.../tests

Yes, CPP tokenized the macro before stringifying it and not surprisingly
the 'linux' part was converted to 1.
[on Fedora-10: cpp (GCC) 4.3.2 20081105 (Red Hat 4.3.2-7)]

So the attached patch simplify the macros and pass the path as string
from the Makefile.

Manually backported from 1449:8ad2773e7ae3

# Compute powerpc-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

    # The kernel ARCH:

    CT_KERNEL_ARCH=powerpc

    # Add spe in the tuplet if needed

    case "${CT_LIBC},${CT_ARCH_POWERPC_SPE}" in

        glibc,|eglibc,)   CT_TARGET_SYS=gnu;;

        glibc,y|eglibc,y) CT_TARGET_SYS=gnuspe;;

    esac

    # Add extra flags for SPE if needed

    if [ "${CT_ARCH_POWERPC_SPE}" = "y" ]; then

        CT_ARCH_TARGET_CFLAGS="-mabi=spe -mspe"

        CT_ARCH_CC_CORE_EXTRA_CONFIG="--enable-e500_double"

        CT_ARCH_CC_EXTRA_CONFIG="--enable-e500_double"

    fi

}