add GNU_FTP_SOURCE template constant

`GNU_SOURCE` (https://ftpmirror.gnu.org/gnu) is sometimes unreachable.
We can use the main source instead of the mirror: https://ftp.gnu.org/gnu
Add the constant `GNU_FTP_SOURCE` for that.
Recommended use:
    `source_urls = [GNU_SOURCE, GNU_FTP_SOURCE]`

easybuild/framework/easyconfig/templates.py

@@ -130,7 +130,9 @@
     ('GNU_SAVANNAH_SOURCE', 'https://download-mirror.savannah.gnu.org/releases/%(namelower)s',
      'download.savannah.gnu.org source url'),
     ('GNU_SOURCE', 'https://ftpmirror.gnu.org/gnu/%(namelower)s',
-     'gnu.org source url'),
+     'gnu.org source url (ftp mirror)'),
+    ('GNU_FTP_SOURCE', 'https://ftp.gnu.org/gnu/%(namelower)s',
+     'gnu.org source url (main ftp)'),
     ('GOOGLECODE_SOURCE', 'http://%(namelower)s.googlecode.com/files',
      'googlecode.com source url'),
     ('LAUNCHPAD_SOURCE', 'https://launchpad.net/%(namelower)s/%(version_major_minor)s.x/%(version)s/+download/',