AUTOMAKE: Disable portability warnings
    
    We already require GNU make extenstions to build
    manual pages.
    
    src/man/Makefile.am:46: warning: wildcard $(srcdir: non-POSIX variable name
    src/man/Makefile.am:46: (probably a GNU make extension)
    src/man/Makefile.am:125: warning: wildcard $(srcdir: non-POSIX variable name
    src/man/Makefile.am:125: (probably a GNU make extension)
    src/man/Makefile.am:128: warning: addprefix $(srcdir: non-POSIX variable name
    src/man/Makefile.am:128: (probably a GNU make extension)
    src/man/Makefile.am:128: warning: shell grep '\[type:docbook\]' $(PO4A_CONFIG: non-POSIX variable name
    src/man/Makefile.am:128: (probably a GNU make extension)
    src/man/Makefile.am:129: warning: filter-out $(CFG_PAGES: non-POSIX variable name
    src/man/Makefile.am:129: (probably a GNU make extension)
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>