The libdir variable is hardcoded in src/libmongoc/CMakeLists.txt and cannot be change using cmake parameter, it is used in the *.pc.in files and as a result on distributions that have different libdir for 64bit like Redhat/Fedora/... pkg-config add a wrong library directory to LDFLAGS.

$ rpm -q mongo-c-driver-devel
mongo-c-driver-devel-1.13.1-1.fc29.x86_64

$ rpm -ql mongo-c-driver-devel  | grep libmongoc-1.0.so
/usr/lib64/libmongoc-1.0.so

$ pkg-config --libs libmongoc-1.0
-L/usr/lib -lmongoc-1.0 -lbson-1.0

$ pkg-config --libs libmongoc-ssl-1.0
-L/usr/lib -lmongoc-1.0 -lbson-1.0