glog assumes that if gflags is available, it has been built with GFLAGS_USE_TARGET_NAMESPACE=OFF

[bucurb]

On this line:

glog/CMakeLists.txt

Line 633 in 42ce901

target_link_libraries (glog PUBLIC gflags)

the glog CMake target adds a dependency to the gflags CMake target. The latter is only available if gflags has been built with GFLAGS_USE_TARGET_NAMESPACE=OFF. Otherwise, the correct target is gflags::gflags. This is causing linker errors, see for instance this issue.