app-admin/lnav: Backport to build with gcc-11

This commit partially[0] backports a patch from upstream[1][2] to
allow lnav to build with gcc-11.

This patch was tested with gcc-10.2.0-r5 and gcc-11.1.0.

[0] Only the changes to m4/ax_cxx_compile_stdcxx.m4 were necessary
    to address the gcc-11 fix.
[1] tstack/lnav#864
[2] tstack/lnav@8f7b08c

Closes: https://bugs.gentoo.org/786456
Package-Manager: Portage-3.0.18, Repoman-3.0.2
Signed-off-by: Randy Barlow <[email protected]>
Closes: gentoo#20975
Signed-off-by: Sam James <[email protected]>