build: Fixes for cross-compilation. (#1527)

This commit includes two changes:

*   Removes existing `AC_NO_EXECUTABLES` from `configure.ac`, since it
    is a hack only intended to be used by the GCC project.
*   Adds a new configure argument: `--enable-no-executables-hack`,
    which restores `AC_NO_EXECUTABLES` for broken build environemnts
    which require it.
*   Removes `--target` from example makefiles, since it is only
    appropriate when compiling compilers.

As a side effect of the second change, cross compiling with the
makefiles now no longer causes the `arm-none-eabi-` prefix to be
prepended to all built executables. This change updates all of the
documentation accordingly.

examples/apps/windows/README.md

@@ -23,4 +23,4 @@ The server listens to the clients.
 
 The client sends a message to the server and the server echos that message back to the client.
 
-![Talk Functionality](../../../doc/images/windows-app-talk-client.png)
+![Talk Functionality](../../../doc/images/windows-app-talk-client.png)