Fix pkgconfig install to respect DESTDIR

The install script for the pkgconfig file was writing directly to
CMAKE_INSTALL_PREFIX without respecting DESTDIR. This broke staged
installs used by package managers (dpkg-buildpackage, rpmbuild, etc).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: revmischa

projectm-eval/install-pkgconfig.cmake.in

@@ -4,4 +4,7 @@ set(CMAKE_INSTALL_LIBDIR "@CMAKE_INSTALL_LIBDIR@")
 set(CMAKE_INSTALL_INCLUDEDIR "@CMAKE_INSTALL_INCLUDEDIR@")
 set(PROJECT_VERSION "@PROJECT_VERSION@")
 set(PROJECTM_EVAL_FLOAT_SIZE "@PROJECTM_EVAL_FLOAT_SIZE@")
-configure_file(@CMAKE_CURRENT_SOURCE_DIR@/projectm-eval.pc.in ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig/projectm-eval.pc @ONLY)
+
+set(PKGCONFIG_INSTALL_DIR "$ENV{DESTDIR}${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+file(MAKE_DIRECTORY "${PKGCONFIG_INSTALL_DIR}")
+configure_file(@CMAKE_CURRENT_SOURCE_DIR@/projectm-eval.pc.in "${PKGCONFIG_INSTALL_DIR}/projectm-eval.pc" @ONLY)