CMakeLists: make daedalus-fourier.pc relocatable via ${pcfiledir}

The pkg-config file was generated at *configure* time with
`prefix=${CMAKE_INSTALL_PREFIX}`, which captured whatever
CMAKE_INSTALL_PREFIX happened to be set to at `cmake -B build`
time — typically the default `/usr/local`.  `cmake --install
build --prefix /foo` then put the files under /foo but the .pc
still pointed pkg-config at /usr/local/include and /usr/local/lib,
which broke downstream consumers configuring against the install
tree.

Concrete bite encountered today on hertz: the daedalus-v4l2 daemon
CMake configure on a /tmp/df-prefix install tree resolved
DAEDALUS_FOURIER_INCLUDE_DIRS to /usr/local/include (empty path on
the test host), so main.c failed to find <daedalus.h>.

Fix: write the .pc with `prefix=${pcfiledir}/<rel>` where <rel> is
the configure-time-computed relative path from
<prefix>/<libdir>/pkgconfig back to <prefix>.  pkg-config
substitutes ${pcfiledir} with the actual on-disk location of the
.pc at lookup time, so the resolved prefix tracks wherever the
install tree is moved to — including DESTDIR-staged builds, apt
package installs, and ad-hoc `cmake --install --prefix /tmp/foo`
test installs.

The relative-path computation handles GNUInstallDirs layouts that
add multiarch tuples (Debian's lib/aarch64-linux-gnu) without
hard-coding `../..`.  Tested on hertz (Debian trixie, libdir=lib):

  prefix=${pcfiledir}/../../
  ...
  $ pkg-config --variable=prefix daedalus-fourier
  /tmp/df-prefix-test/lib/pkgconfig/../../

  # mv preserves relocation:
  $ mv /tmp/df-prefix-test /tmp/df-prefix-moved
  $ pkg-config --variable=prefix daedalus-fourier
  /tmp/df-prefix-moved/lib/pkgconfig/../../

This unblocks the daedalus-v4l2 daemon out-of-tree builds against
local daedalus-fourier installs and is a prerequisite for tidy
test-rig deployments (per the hertz reload session 2026-05-23).

CMakeLists.txt

@@ -419,9 +419,33 @@ endif()
 # pkg-config file.  Vulkan goes in Requires.private (consumer's
 # pkg-config call gets it via --static).  pthread + dl are needed
 # by the static archive's runtime helpers.
+#
+# `prefix` is derived from ${pcfiledir} so the .pc is relocatable:
+# pkg-config substitutes ${pcfiledir} with the directory holding the
+# .pc at lookup time, and the relative path from
+# <prefix>/<libdir>/pkgconfig back to <prefix> tells pkg-config the
+# install prefix without baking it in.  This is why
+# `cmake --install build --prefix /foo` produces a .pc that correctly
+# resolves `prefix=/foo` instead of baking whatever CMAKE_INSTALL_PREFIX
+# was at *configure* time (default /usr/local).  DESTDIR-staged
+# installs work too: at runtime pkg-config sees the .pc at its real
+# install path and computes the right prefix.
+#
+# Relative-path depth is computed from CMAKE_INSTALL_LIBDIR (and
+# whatever multiarch tuple GNUInstallDirs adds) so Debian-style
+# `lib/aarch64-linux-gnu/pkgconfig/...` resolves with the right number
+# of `..` components.  Layouts where libdir is *not* under prefix are
+# not supported by this scheme; if a packager overrides libdir to an
+# absolute path the relative-path machinery falls back to the absolute
+# value (CMake's file(RELATIVE_PATH) prepends `..` until they meet),
+# which is also relocatable but no longer prefix-agnostic.
+file(RELATIVE_PATH PKGCONFIG_PCDIR_TO_PREFIX
+    "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig"
+    "${CMAKE_INSTALL_PREFIX}")
+
 set(PKGCONFIG_OUT ${CMAKE_CURRENT_BINARY_DIR}/daedalus-fourier.pc)
 file(WRITE ${PKGCONFIG_OUT}
-"prefix=${CMAKE_INSTALL_PREFIX}
+"prefix=\${pcfiledir}/${PKGCONFIG_PCDIR_TO_PREFIX}
 exec_prefix=\${prefix}
 libdir=\${prefix}/${CMAKE_INSTALL_LIBDIR}
 includedir=\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}