debug/cross-gdb: check host dependencies

Cross-gdb depends on expat and python. If either is missing, cross-gdb will
build successfully, but lacking some features.

Especially, if expat is missing, cross-gdb will be unable to parse the target
description, which may lead to runtime malfunctions and the following GDB
warning:
"Can not parse XML target description; XML support was disabled at compile time"

Hence, expat should be considered mandatory.

On the other hand, the features missing without python are not critical, so
python should not be considered mandatory.

This patch does the following:
- At configure time, warn the user if either expat or python is missing.
- In menuconfig, disable the static build options regarding cross-gdb if no
static version of expat is available, and disable cross-gdb if expat is
missing.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>
[yann.morin.1998@anciens.enib.fr: add comment for impossible static cross-gdb]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# powerpc specific configuration file

config ARCH_POWERPC_SPE

    bool

    prompt "Enable SPE support"

    help

      Add support for the Signal Processing Engine.  This will set up

      the toolchain so that it supports the SPE ABI extensions. This

      mainly targets Freescale e500 processors.

      Setting this option will append "spe" to the end of your target

      tuple name (e.g., powerpc-e500v2-linux-gnuspe) so that the gcc

      configure/build system will know to include SPE ABI support. It

      will also automatically add "-mabi=spe -mspe" to your TARGET_CFLAGS,

      and "--enable-e500_double" to your CC_EXTRA_CONFIG_ARRAY, so you

      do not need to explicitly add them.