emerge -a1 strongswan
These are the packages that would be merged, in order:
Calculating dependencies... done!
[ebuild   R    ] net-vpn/strongswan-5.7.2

/bin/sh ../../libtool  --tag=CC   --mode=link x86_64-pc-linux-gnu-gcc  -march=westmere -O2 -pipe -include /var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2/config.h  -Wl,-O1 -Wl,--sort-common -Wl,--as-needed -o swanctl command.o commands/counters.o commands/initiate.o commands/terminate.o commands/rekey.o commands/redirect.o commands/install.o commands/list_sas.o commands/list_pols.o commands/list_authorities.o commands/list_conns.o commands/list_certs.o commands/list_pools.o commands/list_algs.o commands/flush_certs.o commands/load_all.o commands/load_authorities.o commands/load_conns.o commands/load_creds.o commands/load_pools.o commands/log.o commands/version.o commands/stats.o commands/reload_settings.o swanctl.o ../../src/libcharon/plugins/vici/libvici.la ../../src/libstrongswan/libstrongswan.la -lpthread -ldl   -ldl libtool: link: x86_64-pc-linux-gnu-gcc -march=westmere -O2 -pipe -include /var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2/config.h -Wl,-O1 -Wl,--sort-common -Wl,--as-needed -o .libs/swanctl command.o commands/counters.o commands/initiate.o commands/terminate.o commands/rekey.o commands/redirect.o commands/install.o commands/list_sas.o commands/list_pols.o commands/list_authorities.o commands/list_conns.o commands/list_certs.o commands/list_pools.o commands/list_algs.o commands/flush_certs.o commands/load_all.o commands/load_authorities.o commands/load_conns.o commands/load_creds.o commands/load_pools.o commands/log.o commands/version.o commands/stats.o commands/reload_settings.o swanctl.o  ../../src/libcharon/plugins/vici/.libs/libvici.so ../../src/libstrongswan/.libs/libstrongswan.so -lpthread -ldl -Wl,-rpath -Wl,/usr/lib64/ipsec /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: commands/load_authorities.o:(.bss+0x0): multiple definition of `swanctl_dir'; commands/load_all.o:(.bss+0x0): first defined here /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: commands/load_conns.o:(.bss+0x0): multiple definition of `swanctl_dir'; commands/load_all.o:(.bss+0x0): first defined here /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: commands/load_creds.o:(.bss+0x0): multiple definition of `swanctl_dir'; commands/load_all.o:(.bss+0x0): first defined here /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: commands/load_pools.o:(.bss+0x0): multiple definition of `swanctl_dir'; commands/load_all.o:(.bss+0x0): first defined here /usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../x86_64-pc-linux-gnu/bin/ld: swanctl.o:(.bss+0x0): multiple definition of `swanctl_dir'; commands/load_all.o:(.bss+0x0): first defined here collect2: error: ld returned 1 exit status make[4]: *** [Makefile:663: swanctl] Error 1 make[4]: Leaving directory '/var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2/src/swanctl' make[3]: *** [Makefile:520: all] Error 2 make[3]: Leaving directory '/var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2/src/swanctl' make[2]: *** [Makefile:530: all-recursive] Error 1 make[2]: Leaving directory '/var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2/src' make[1]: *** [Makefile:593: all-recursive] Error 1 make[1]: Leaving directory '/var/tmp/portage/net-vpn/strongswan-5.7.2/work/strongswan-5.7.2' make: *** [Makefile:504: all] Error 2