pkg-conf file has arch-dependent contents but is installed to arch-independent datadir

[drew-parsons]

The destination for the pkg-conf file xsimd.pc at

xsimd/CMakeLists.txt

Line 167 in e0a1058

DESTINATION "${CMAKE_INSTALL_DATADIR}/pkgconfig/")

was changed in PR #1026 to an arch-independent datadir (/usr/share/pkgconfig).

Using the datadir is correct for the cmake config files (in /usr/share/cmake), since xsimdConfig.cmake and xsimdTargets.cmake now have no arch-dependent references.

But xsimd.pc (xsimd.pc.in) contains libdir, which in general is arch-dependent, at least on systems which use GnuInstallDirs where, for instance, libdir= /usr/lib/x86_64-linux-gnu on an amd64 system (and xsimd does invoke GNUInstallDirs in CMakeLists.txt). I think in that case the general rule is that xsimd.pc ought to continue to be installed under libdir, not datadir.

For that reason I think PR #1026 should be half reverted to put xsimd.pc back under libdir.

Alternatively, since xsimd is header only as PR #1026 points out, would it be better to remove the libdir entry from xsimd.pc.in (or left empty), since xsimd is not actually providing any libraries in libdir? I'm not sure about the strict rules for the pkg-config file format. Does libdir need to be set, or need to point at the arch-dependent libdir?