After using the feeds script to add strongswan to my build directory, make menuconfig shows a few different selection lists for strongswan like strongswan-default and strongswan-minimal. I liked strongswan-isakmp, so I chose that.  However that selects strongswan-mod-gmpdh which will currently fail to build without strongswan-mod-gmp:

../gmp/gmp_diffie_hellman.c:20:17: fatal error: gmp.h: No such file or directory
compilation terminated.

So I just select strongswan-mod-gmp to complete the build. Just thought I would mention that here.  Hope you have a great day!