Fix LTO check on FreeBSD11

[erlbeck]

Currently the LTO (link time optimisation) check is done by trying to compile a single object file with the -flto option. This way the check does not cover the case that the compiler supports it but the linker does not.

This can happen for instance on FreeBSD11 with FreeBSD clang version 6.0.0 in combination with GNU ld 2.17.50, where the latter does not support the -plugin option.

Modify the check by removing the -c flag from the compiler options.

[dch]

@davisp I can't say I fully understand the issue, but I run into this on FreeBSD atm since last clang updates. Can you +1 / merge & tag so we can get this into couchdb git please?