Add a more common and more versatile example to the Verbatim C-Code documentation.

docs/examples/userguide/external_C_code/platform_adaptation.pyx

+cdef extern from *:
+    """
+    #if defined(WIN32) || defined(MS_WINDOWS)
+      #include "windows.h"
+      #define myapp_sleep(m)  Sleep(m)
+    #else
+      #include <unistd.h>
+      #define myapp_sleep(m)  ((void) usleep((m) * 1000))
+    #endif
+    """
+    # using "myapp_" prefix in the C code to prevent C naming conflicts
+    void msleep "myapp_sleep"(int milliseconds) nogil
+
+msleep(milliseconds=1)

docs/src/userguide/external_C_code.rst

@@ -335,7 +335,7 @@ Including verbatim C code
 For advanced use cases, Cython allows you to directly write C code
 as "docstring" of a ``cdef extern from`` block:
 
-.. literalinclude:: ../../examples/userguide/external_C_code/c_code_docstring.pyx
+.. literalinclude:: ../../examples/userguide/external_C_code/verbatim_c_code.pyx
 
 The above is essentially equivalent to having the C code in a file
 ``header.h`` and writing ::
@@ -344,6 +344,11 @@ The above is essentially equivalent to having the C code in a file
         long square(long x)
         void assign(long& x, long y)
 
+This feature is commonly used for platform specific adaptations at
+compile time, for example::
+
+.. literalinclude:: ../../examples/userguide/external_C_code/platform_adaptation.pyx
+
 It is also possible to combine a header file and verbatim C code::
 
     cdef extern from "badheader.h":