Makefile.autotools.in: fix DESTDIR= help text

#	install the package to the staging directory

# FOO_INSTALL_TARGET [YES/NO, default YES]

#	install the package to the target directory

# FOO_INSTALL_STAGING_OPT [default DESTDIR=$(STAGING_DIR)/usr install]

# FOO_INSTALL_STAGING_OPT [default DESTDIR=$(STAGING_DIR) install]

#	arguments passed to <make> while installing to the staging directory

# FOO_INSTALL_TARGET_OPT [default DESTDIR=$(TARGET_DIR)/usr install-exec]

# FOO_INSTALL_TARGET_OPT [default DESTDIR=$(TARGET_DIR) install-exec]

#	arguments passed to <make> while installing to the target directory

# FOO_CLEAN_OPT [default clean]

#	arguments passed to <make> while installing to the staging directory

# FOO_UNINSTALL_STAGING_OPT  [default DESTDIR=$(STAGING_DIR)/usr uninstall]

# FOO_UNINSTALL_STAGING_OPT  [default DESTDIR=$(STAGING_DIR) uninstall]

#	arguments passed to <make> while uninstalling from the staging

#	directory

# FOO_UNINSTALL_TARGET_OPT [default DESTDIR=$(TARGET_DIR)/usr uninstall]

# FOO_UNINSTALL_TARGET_OPT [default DESTDIR=$(TARGET_DIR) uninstall]

#	arguments passed to <make> while uninstalling from the target

#	directory

# FOO_SUBDIR [default empty]