pybind11: cmake: ignore the check between host-python and cross-compiler (#1848)

When dlib is compiling, cmake will compare python architecture and target
architecture. So in cross-compiling case, it is irrevelant because host and
target architecture often differs. The main problem come from checking python
architecture on host and not on target.

Here is an error when compiling dlib from x86_64 to arm 32-bit target :
```
Python config failure: Python is 64-bit, chosen compiler is 32-bit
```

So :
- Skipping the comparation when cross-compiling is enabled.
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Alexandre PAYEN <alexandre.payen@smile.fr>

dlib/external/pybind11/tools/FindPythonLibsNew.cmake

@@ -113,18 +113,21 @@ list(GET _PYTHON_VALUES 7 PYTHON_LIBRARY_SUFFIX)
 list(GET _PYTHON_VALUES 8 PYTHON_LIBDIR)
 list(GET _PYTHON_VALUES 9 PYTHON_MULTIARCH)
 
-# Make sure the Python has the same pointer-size as the chosen compiler
-# Skip if CMAKE_SIZEOF_VOID_P is not defined
-if(CMAKE_SIZEOF_VOID_P AND (NOT "${PYTHON_SIZEOF_VOID_P}" STREQUAL "${CMAKE_SIZEOF_VOID_P}"))
-    if(PythonLibsNew_FIND_REQUIRED)
-        math(EXPR _PYTHON_BITS "${PYTHON_SIZEOF_VOID_P} * 8")
-        math(EXPR _CMAKE_BITS "${CMAKE_SIZEOF_VOID_P} * 8")
-        message(FATAL_ERROR
-            "Python config failure: Python is ${_PYTHON_BITS}-bit, "
-            "chosen compiler is  ${_CMAKE_BITS}-bit")
+# Ignore this test while crosscompiling otherwise it will use the host python.
+IF(NOT CMAKE_CROSSCOMPILING)
+    # Make sure the Python has the same pointer-size as the chosen compiler
+    # Skip if CMAKE_SIZEOF_VOID_P is not defined
+    if(CMAKE_SIZEOF_VOID_P AND (NOT "${PYTHON_SIZEOF_VOID_P}" STREQUAL "${CMAKE_SIZEOF_VOID_P}"))
+        if(PythonLibsNew_FIND_REQUIRED)
+            math(EXPR _PYTHON_BITS "${PYTHON_SIZEOF_VOID_P} * 8")
+            math(EXPR _CMAKE_BITS "${CMAKE_SIZEOF_VOID_P} * 8")
+            message(FATAL_ERROR
+                "Python config failure: Python is ${_PYTHON_BITS}-bit, "
+                "chosen compiler is  ${_CMAKE_BITS}-bit")
+        endif()
+        set(PYTHONLIBS_FOUND FALSE)
+        return()
     endif()
-    set(PYTHONLIBS_FOUND FALSE)
-    return()
 endif()
 
 # The built-in FindPython didn't always give the version numbers