/usr/lib/pkgconfig/xorg-synaptics.pc is in xserver-xorg-input-synaptics-dev 1.5.99.902-0ubuntu5.
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 | sdkdir=/usr/include/xorg
Name: synaptics
Description: X.Org synaptics input driver.
Version: 1.5.99.902
Cflags: -I${sdkdir}
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