Update CMakeLists.txt

Co-authored-by: Miloš Komarčević <4973094+kmilos@users.noreply.github.com>

CMakeLists.txt

@@ -984,9 +984,17 @@ endif()
 # Create pkgconfig files.
 # We use the same files like ./configure, so we have to set its vars.
 set(prefix      ${CMAKE_INSTALL_PREFIX})
-set(exec_prefix ${CMAKE_INSTALL_PREFIX})
-set(libdir      ${CMAKE_INSTALL_FULL_LIBDIR})
-set(includedir  ${CMAKE_INSTALL_FULL_INCLUDEDIR})
+set(exec_prefix "\${prefix}")
+if(IS_ABSOLUTE "${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}/${CMAKE_INSTALL_INCLUDEDIR}")
+endif()
 set(LIBS        "-lz -lm")
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng.pc.in
                ${CMAKE_CURRENT_BINARY_DIR}/libpng${PNGLIB_ABI_VERSION}.pc