Make sure yahttp is linked into remotebackend

Make sure that the yahttp directory is built first.

Adding the archive causes the linker to do this:
libtool: link: g++  -fPIC -DPIC -shared -nostdlib /usr/lib/gcc/x86_64-redhat-linux/4.9.1/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/4.9.1/crtbeginS.o  .libs/remotebackend.o .libs/unixconnector.o .libs/httpconnector.o .libs/pipeconnector.o .libs/zmqconnector.o  -Wl,--whole-archive ../../pdns/ext/yahttp/yahttp/.libs/libyahttp.a -Wl,--no-whole-archive  -lrt -L/usr/lib/gcc/x86_64-redhat-linux/4.9.1 -L/usr/lib/gcc/x86_64-redhat-linux/4.9.1/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.9.1/../../.. -lstdc++ -lm -lc -lgcc_s /usr/lib/gcc/x86_64-redhat-linux/4.9.1/crtendS.o /usr/lib/gcc/x86_64-redhat-linux/4.9.1/../../../../lib64/crtn.o  -O2 -Wl,-z -Wl,relro -Wl,-z -Wl,now   -Wl,-soname -Wl,libremotebackend.so -o .libs/libremotebackend.so

Note the --whole-archive stuff.

Fixes #1732