/usr/lib/x86_64-linux-gnu/pkgconfig/libmodbus.pc is in libmodbus-dev 3.0.6-2.
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 | prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib/x86_64-linux-gnu
includedir=${prefix}/include
Name: modbus
Description: Modbus library
Version: 3.0.6
Libs: -L${libdir} -lmodbus
Cflags: -I${includedir}/modbus
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