Switch to GNU mirrors for GNU dependencies downloads

[vemcaster]

Switch to GNU Mirrors for GNU dependencies downloads

Problem

Encountered timeouts and critically low download speeds from ftp.gnu.org when building Docker images from CI locations in Sweden, Germany, and Cyprus. This resulted in build failures like this:

#9 50.56 building m4 - version 1.4.19
#9 50.56 =======================
#9 50.56 Downloading https://ftp.gnu.org/gnu/m4/m4-1.4.19.tar.gz as m4-1.4.19.tar.gz
#9 184.5 
#9 184.5 Failed to download https://ftp.gnu.org/gnu/m4/m4-1.4.19.tar.gz. Exitcode 28. Retrying in 10 seconds
#9 327.9 
#9 327.9 Failed to download https://ftp.gnu.org/gnu/m4/m4-1.4.19.tar.gz. Exitcode 28
#9 ERROR: process "/bin/sh -c SKIPINSTALL=yes /app/build-ffmpeg --build --full-static --enable-gpl-and-non-free --small" did not complete successfully: exit code: 1

Solution

Switched to using GNU mirrors ftpmirror.gnu.org for downloading GNU components.

Rationale

• GNU officially recommends using mirrors for downloads (see GNU Mirror List documentation)
• Some components in the codebase already use ftpmirror.gnu.org, making this change consistent with existing practices
• Mirror infrastructure provides better geographic distribution and redundancy

Changes

Updated download URLs for the following GNU components:

• m4
• autoconf
• automake
• gmp
• nettle

All downloads now use https://ftpmirror.gnu.org/gnu/ instead of https://ftp.gnu.org/gnu/.

Testing

• Verified builds complete successfully from multiple geographic locations (Cyprus, Germany, Sweden)
• All component versions remain unchanged