Many thanks. strongswan is also included in openwrt-sdk under feeds/packages/net/strongswan. I attempted to build it using:
`$ make package/strongswan/compile`
but this failed attempting to build libgcrypt because gpg-error-config was missing from the host system. I then installed libgpg-error-dev using:
`$ sudo apt-get install libgpg-error-dev`
which provided /usr/bin/gpg-error-config. This failed again because the symlink staging_dir/target-mipsel_mips32_musl-1.1.14/usr/bin/gpg-error-config pointed to /bin/gpg-error-config. I added a symlink with the following:
`$ sudo ln -s /usr/bin/gpg-error-config /bin/gpg-error-config`
I was then able to complete the build of the strongswan packages.
I removed the symlink and rebuilt using a clean copy of open-sdk and the problem didn't recur. So to build strongswan ensure that the libgpg-error-dev package is installed on the host system before installing open-sdk.
Thanks again for the help.