CMake: align .pc variables with autoconf

Use prefix/exec_prefix for includedir/libdir if
CMAKE_INSTALL_{LIBDIR,INCLUDEDIR} are relative and the CMake variable
alone if they're absolute.

This matches what autoconf will produce if a custom --libdir or
--includedir are used.

See also: AOMediaCodec/libavif#1220

Change-Id: I6fbc016c0009ede42e1e5db3a9ee7ae9545d8da0

CMakeLists.txt

@@ -128,9 +128,17 @@ endif()
 
 # pkg-config variables used by *.pc.in.
 set(prefix ${CMAKE_INSTALL_PREFIX})
-set(exec_prefix "\$\{prefix\}")
-set(libdir "\$\{prefix\}/lib")
-set(includedir "\$\{prefix\}/include")
+set(exec_prefix "\${prefix}")
+if(IS_ABSOLUTE "${CMAKE_INSTALL_LIBDIR}")
+  set(libdir "${CMAKE_INSTALL_LIBDIR}")
+else()
+  set(libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
+endif()
+if(IS_ABSOLUTE "${CMAKE_INSTALL_INCLUDEDIR}")
+  set(includedir "${CMAKE_INSTALL_INCLUDEDIR}")
+else()
+  set(includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+endif()
 set(PTHREAD_LIBS ${CMAKE_THREAD_LIBS_INIT})
 set(INSTALLED_LIBRARIES)