fix configure for cross-compilation

[ayoubserti]

This pull request fix a cross-compilation issue:
When I try to configure for cross-compilation the script fails on wcsrtombs test.

[FerrixPlusPlus]

I have tested the fix on my environment ( Debian 10 with crossbuild-essential-armhf package ) and works fine.

[ayoubserti]

Thank you @FerrixPlusPlus
Still the maintainer approval.

[rleigh-codelibre]

Do you need to repeat the program source code? Or would AC_CHECK_FUNC be sufficient when cross-compiling?

Do the CMake checks in cmake/XercesWChar.cmake also need updating to cater for this scenario?

[ayoubserti]

@rleigh-codelibre I'm not sure if AC_CHECK_FUNC compile and run or only compile.
In general, in cross-compiling the configure should use AC_COMPILE_IFELSE instead of AC_RUN_IFELSE

cmake/XercesWChar.cmake check compilation only