There is no pkg-config file which would be very
convenient for many config tools

Here is one that could be used on systems that have
the pkg-config tool:
(Requires may need tunning. lib64 may be replaced with lib
on 32bits systems).

Filename: /usr/lib64/pkgconfig/xm.pc
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prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
includedir=/usr/include

Name: Xm
Description: Open Motif Widgets Library, version 2.3.x
Version: 2.3.3
Requires: xproto xt xmu
Requires.private: x11 xext xt xmu xpm
Cflags: -I${includedir}
Libs: -L${libdir} -lXm
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