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>

# Test suite config options

if EXPERIMENTAL

menu "Test suite"

config TEST_SUITE

    bool

config TEST_SUITE_GCC

    bool

    prompt "GCC test suite"

    depends on EXPERIMENTAL

    select TEST_SUITE

    help

      Select this option to install the GCC test suite in $CT_PREFIX_DIR/test_suite.

      The GCC test suite includes a collection of various toolchain tests for GCC -

      it utilizes the DejaGnu test framework.

      For some tests a network enabled target with ssh server is required.

      A helper Makefile is provided for running the tests - please see the included

      README for information on how to run the test suite.

endmenu

endif