httping: also pass configuration (SSL/TFO/DEBUG) doing make install

Fixes http://autobuild.buildroot.net/results/144e146ad3223748c4f06f884aeff93e8c0c2de3

When SSL is enabled (which it is by default) an extra source file is built,
so ensure the same configuration is used at 'make install' time as during
the build step.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>