build: don't call functions we didn't detect to exist
build: don't call functions we didn't detect to exist

In case of a mismatch between headers and libs (uuid/uuid.h found,
uuid_generate not found) make sure we rely on the latter, or else the
linking stage will fail
http://monetdb.cwi.nl/testweb/logs/43264:7ab4f1e62759/GNU-Fedora-i386-propcheck/make.html#l2039

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb