civetweb: unbreak static linking builds

Fixes
http://autobuild.buildroot.net/results/0a5/0a555a4ad0d100a7d402908b1f26d213de0ec127/
http://autobuild.buildroot.net/results/97b/97b014b2a611ba2528dda86bb05d6ffbec51c21d/
http://autobuild.buildroot.net/results/c2e/c2e314127be74a41bcdb78f04bc51fe2edef0b0f/
http://autobuild.buildroot.net/results/734/7346a89b5e7dd3633ecf9ac5292379ec9e832b2e/



And many more.

It is only the lua suboption that needs dlopen support, so adjust the
toolchain dependencies and CIVETWEB_LIBS handling to match.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>