gtkdocize.in: ensure that m4 directory exists

Some packages set AC_CONFIG_MACRO_DIR without shipping macros. These end up in errors
like

| ln: target 'm4/' is not a directory: No such file or directory
| cp: cannot create regular file 'm4/': Not a directory

Note: This error does not occure e.g when libtoolize is run before because it
creates macro directory.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>