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 Gentoo.

 -= BEGIN original header =-

 ARMV5 can use STRD and LDRD access instructions but these accesses need to be 

 8 byte aligned. The dynamic linker's malloc needs to match this so structures

 become 8 byte aligned to void unaligned accesses.

 RP - 14/02/2008

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/ldso/ldso/arm/dl-sysdep.h uClibc-0.9.30/ldso/ldso/arm/dl-sysdep.h

 --- uClibc-0.9.30.orig/ldso/ldso/arm/dl-sysdep.h	2008-09-25 10:35:20.000000000 +0200

 +++ uClibc-0.9.30/ldso/ldso/arm/dl-sysdep.h	2009-02-07 09:57:59.000000000 +0100

 @@ -15,6 +15,8 @@

    GOT_BASE[1] = (unsigned long) MODULE; \

  }

 +#define DL_MALLOC_ALIGN 8  /* EABI needs 8 byte alignment for STRD LDRD*/

 +

  static __inline__ unsigned long arm_modulus(unsigned long m, unsigned long p)

  {

  	unsigned long i,t,inc;