[BUG] TCP_FASTOPEN fails to be enabled on supported kernels such as 6.1 and 6.6.

[BeardOverflow]

Description

TCP_FASTOPEN fails to be enabled on supported kernels such as 6.1 and 6.6.

Steps to Reproduce

1. Check your current kernel
   a) Use a real kernel such as 6.1 or 6.6
   b) Or fake the installed kernel version in your system:

   1. Backup your /bin/uname (revert it at the end): mv /bin/uname /bin/uname.bak
   2. Create a fake /bin/uname binary with the following content:

   #!/bin/bash
   
   case "$1" in
     -m) echo "x86_64";;
     -r) echo "6.1.0-amd64";;
     -s) echo "Linux";;
     -v) echo "#1 SMP PREEMPT_DYNAMIC";;
     *) echo "Linux";;
   esac
   

   3. Make sure the new /bin/uname has execution permissions: chmod +x /bin/uname

2. Follow the building steps in the README.md to configure the project:

./bootstrap
mkdir build
cd build
../configure

Expected behavior:
TCP_FASTOPEN to yes

Actual behavior:
TCP_FASTOPEN to no

configure: Configuration Summary:
  Operating System:  linux-gnu
  Target directory:  /usr/local
  License         :  LGPL only
  Debug           :  no
  TLS Enabled     :  yes
  TCP_FASTOPEN    :  no
  Static          :  yes
  Build examples  :  yes

Reproduces how often: Always

Versions

• Linux pc 6.1.0-32-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.129-1 (2025-03-06) x86_64 GNU/Linux
• libhttpserver version (compiled): https://github.com/etr/libhttpserver/tree/dff6af2fc17f9ac313e64ab8fb83bd01a1d7da70
• libmicrohttpd version (compiled): https://ftpmirror.gnu.org/libmicrohttpd/libmicrohttpd-1.0.2.tar.gz

Additional Information

None