gflags: Add new wrap

[WillAyd]

This is a pre-cursor to add support for Apache Arrow in #2099

[bgilbert]

This creates a subdirectory src/gflags that doesn't exist in the upstream repo, which works but is a bit confusing. Since everything here specifies install_dir: 'gflags', is that actually needed?

[WillAyd]

IIUC the configure_file calls in the CMake configuration create the header files in:

"${PROJECT_BINARY_DIR}/include/${GFLAGS_INCLUDE_DIR}

where the GFLAGS_INCLUDE_DIR is by default set to the name of the package itself. I also am of the understanding that Meson does not give you the control to choose a different build directory to write a configured file into, but that it instead infers it from the location of the meson.build file in the source tree.

So this is a bit of a compromise where we place this configuration somewhere in the source tree so that the default include search patch will still resolve to gflags/<configured_file>.h, but there may be a better way to handle that