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>

 Original patch from: ../4.3.2/350-sh-without-headers.patch

 -= BEGIN original header =-

 On 20081011, Khem RAJ writes:

  You have to bar gcc trying to build unwinding at stage 1. Try the

  attached patch and see if it helps.

  gcc/config/sh/linux-unwind.h |    3 +++

  1 file changed, 3 insertions(+)

 -= END original header =-

 diff -durN gcc-4.3.3.orig/gcc/config/sh/linux-unwind.h gcc-4.3.3/gcc/config/sh/linux-unwind.h

 --- gcc-4.3.3.orig/gcc/config/sh/linux-unwind.h	2007-09-12 08:59:06.000000000 +0200

 +++ gcc-4.3.3/gcc/config/sh/linux-unwind.h	2009-01-27 22:28:57.000000000 +0100

 @@ -26,6 +26,8 @@

  the Free Software Foundation, 51 Franklin Street, Fifth Floor,

  Boston, MA 02110-1301, USA.  */

 +#ifndef inhibit_libc

 +

  /* Do code reading to identify a signal frame, and set the frame

     state data appropriately.  See unwind-dw2.c for the structs.  */

 @@ -251,3 +253,4 @@

    return _URC_NO_REASON;

  }

  #endif /* defined (__SH5__) */

 +#endif /* ifndef inhibit_libc */