It seems that these influence how subsequent tests are performed, and some interaction with a change in autoconf-2.70 caused the test for shm_open() to fail. See the announcement [0] and the "Setting CC to a C++ compiler is no longer supported." section specifically.
These were added in commit 909511c8 ("build: Run autoupdate on configure.ac to remove some obsolete autotools crap"). Running autoupdate-2.71 on configure.ac today with the AC_LANG(...) calls removed does not restore them.
[0] https://lists.gnu.org/archive/html/autoconf/2020-12/msg00002.html
Bug: https://bugs.gentoo.org/817017 Closes: https://gitlab.gnome.org/GNOME/anjuta/-/issues/30