Install Sage-specific .pc files when running make, not configure

[embray]

As noted in ​this comment, if we generate any .pc files at configure time, rather than write them directly to $SAGE_LOCAL it would be preferable to write them somewhere in the sage source tree instead, then copy them into the relevant install target (i.e. $SAGE_LOCAL) only when running make.

One tricky aspect to this is at least some SPGKs require our generated .pc files to be installed in $SAGE_LOCAL to build properly. Currently, one way around that is to add $(PCFILES) to the package's dependencies. This is needed at the very least for numpy.

A different workaround might be to modify PKG_CONFIG_PATH to include .pc files in the source tree. I have not tried that yet.

[embray]

First attempt at this seems to work.

I chose src/lib/pkgconfig as the path for .pc files to install from the source tree, as it has a nice congruence with src/bin. But that was just a minor preference--this path could be anywhere in the source tree, or even outside it (e.g. for VPATH builds, should we ever get that working...)

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New commits:

​16a0a75	Trac #29001: Add workaround for older version of pkg-config that do not include the PKG_CHECK_VAR macro
​d5c9520	Create generated .pc files in the source tree and install them later instead of creating them directly in SAGE_LOCAL

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​4cc7f45

Create generated .pc files in the source tree and install them later instead of creating them directly in SAGE_LOCAL

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​854c899	Trac #29003: Create generated .pc files in the source tree and install them later instead of creating them directly in SAGE_LOCAL
​62ac3cc	Trac #29003: Also output generated gsl.pc to SAGE_SRC
​e59f991	Trac #29003: Instead of making individual packages responsible for .pc files being installed, just include them as part of the standard 'toolchain'

[embray]

Replying to git:

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​854c899	Trac #29003: Create generated .pc files in the source tree and install them later instead of creating them directly in SAGE_LOCAL
​62ac3cc	Trac #29003: Also output generated gsl.pc to SAGE_SRC
​e59f991	Trac #29003: Instead of making individual packages responsible for .pc files being installed, just include them as part of the standard 'toolchain'

I think the approach in this version might be simplest, and is closer to the existing behavior. The major difference now is that actually copying .pc files into $SAGE_LOCAL is deferred until make is run, rather than writing to $SAGE_LOCAL directly when running configure.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​90e81ad	Trac #29003: Also output generated gsl.pc to SAGE_SRC
​c7fbc65	Trac #29003: Instead of making individual packages responsible for .pc files being installed, just include them as part of the standard 'toolchain'

[dimpase]

testing

[dimpase]

OK, good, thanks!

[dimpase]

there is sometimes a permission problem, shouldn't that cp -P used to install these things be replaced by something more robust.

see e.g. ​https://trac.sagemath.org/ticket/29071#comment:8

[dimpase]

by right, these *.pc files are package data, so they ought to be treated as such, no?

[dimpase]

$(PCFILES) must be cleanable, by make clean I guess. Pleadse confirm. It caused a lot of pain on #29071

[dimpase]

the followup (cleaning of *.pc files) is on #29082

[mkoeppe]

... hotfix in #29121