Install espeak-ng.pc under the real libdir, with libdir/includedir taken
from GNUInstallDirs. As shipped at a4ca101c, cmake/package.cmake hardcodes
lib/pkgconfig and ${exec_prefix}/lib, so on lib64 systems pkg-config cannot
find espeak-ng at all. Backport of upstream 35524b3 + 6c3ba4e (2026-04),
both on master; drop when the snapshot moves past them.

diff -ru a/cmake/package.cmake b/cmake/package.cmake
--- a/cmake/package.cmake
+++ b/cmake/package.cmake
@@ -21,8 +21,8 @@
 
 set(prefix ${CMAKE_INSTALL_PREFIX})
 set(exec_prefix ${CMAKE_INSTALL_PREFIX})
-set(libdir "\${exec_prefix}/lib")
-set(includedir "\${prefix}/include")
+set(libdir ${CMAKE_INSTALL_FULL_LIBDIR})
+set(includedir ${CMAKE_INSTALL_FULL_INCLUDEDIR})
 
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/${PROJECT_NAME}.pc.in
@@ -31,5 +31,5 @@
 )
 
 install(
-  FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc DESTINATION lib/pkgconfig
-)
\ No newline at end of file
+  FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+)
diff -ru a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,6 +12,7 @@
 
 include(CTest)
 
+include(GNUInstallDirs)
 include(cmake/deps.cmake)
 include(cmake/config.cmake)
 add_subdirectory(src)
diff -ru a/src/CMakeLists.txt b/src/CMakeLists.txt
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -14,7 +14,7 @@
 set_target_properties(
   espeak-ng-bin PROPERTIES
   MACOSX_RPATH ON
-  INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib"
+  INSTALL_RPATH "${CMAKE_INSTALL_LIBDIR}"
 )
 if (MINGW)
   target_link_options(espeak-ng-bin PRIVATE "-static-libstdc++" "-static")
diff -ru a/src/libespeak-ng/CMakeLists.txt b/src/libespeak-ng/CMakeLists.txt
--- a/src/libespeak-ng/CMakeLists.txt
+++ b/src/libespeak-ng/CMakeLists.txt
@@ -40,7 +40,7 @@
 set_target_properties(
   espeak-ng PROPERTIES
   MACOSX_RPATH ON
-  INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib"
+  INSTALL_RPATH "${CMAKE_INSTALL_LIBDIR}"
 )
 
 target_include_directories(espeak-ng BEFORE PRIVATE $<TARGET_PROPERTY:espeak-include,INTERFACE_INCLUDE_DIRECTORIES>)