Qt4Macros: Make QT4_CREATE_MOC_COMMAND a function

QT4_CREATE_MOC_COMMAND is given a moc_flags argument that contains the
COMPILE_DEFINITIONS and a potentially large list of include directories.
Since it is a macro, the ${moc_flags} reference is replaced with this
content and sent through cmMakefile::ExpandVariablesInString (EVIS).
Since commit v3.0.0-rc1~138^2 (Qt4: Use generator expression in
COMPILE_DEFINITIONS, 2014-01-13) the COMPILE_DEFINITIONS value contains
a '$' so the EVIS fast-path is no longer used.  Instead the full
cmCommandArgumentParserHelper is now used on the large input, which is
very slow (since it was originally created for hand-written code).

Change QT4_CREATE_MOC_COMMAND to a function instead of a macro to avoid
passing large content through EVIS.  This makes it significantly faster.

Modules/Qt4Macros.cmake

@@ -103,7 +103,7 @@ endmacro()
 
 
 # helper macro to set up a moc rule
-macro (QT4_CREATE_MOC_COMMAND infile outfile moc_flags moc_options moc_target)
+function (QT4_CREATE_MOC_COMMAND infile outfile moc_flags moc_options moc_target)
   # For Windows, create a parameters file to work around command line length limit
   # Pass the parameters in a file.  Set the working directory to
   # be that containing the parameters file and reference it by
@@ -144,7 +144,7 @@ macro (QT4_CREATE_MOC_COMMAND infile outfile moc_flags moc_options moc_target)
                       DEPENDS ${infile}
                       ${_moc_working_dir}
                       VERBATIM)
-endmacro ()
+endfunction ()
 
 
 macro (QT4_GENERATE_MOC infile outfile )