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>

 # mingw32 config options

 ## depends on EXPERIMENTAL && ( ARCH_x86 && ARCH_32 )

 ##

 ## select MINGW32

 ##

 ## help Build a toolchain targeting systems running Windows as host

 choice

     bool

     prompt "Windows api version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config W32API_V_3_14

     bool

     prompt "3.14"

 config W32API_V_select

     bool

     prompt "Other version"

 endchoice

 config W32API_VERSION

     string

     prompt "W32 api version" if W32API_V_select

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "3.14" if W32API_V_3_14

     help

       Enter the version number of the windows api files to use