/usr/lib/x86_64-linux-gnu/pkgconfig/xcb-glx.pc is in libxcb-glx0-dev 1.11.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib/x86_64-linux-gnu
includedir=${prefix}/include
Name: XCB GLX
Description: XCB GLX Extension
Version: 1.11.1
Requires: xcb
Libs: -L${libdir} -lxcb-glx
Cflags: -I${includedir}
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