Use GNUInstallDirs variables for install paths.

GNUInstallDirs is already included upstream since 0.20.x but the
OLLAMA_BUILD_DIR and OLLAMA_INSTALL_DIR still hardcode "lib".

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -55,8 +55,8 @@ if(APPLE)
     set(CMAKE_BUILD_WITH_INSTALL_RPATH ON)
 endif()

-set(OLLAMA_BUILD_DIR ${CMAKE_BINARY_DIR}/lib/ollama)
-set(OLLAMA_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/lib/ollama/${OLLAMA_RUNNER_DIR})
+set(OLLAMA_BUILD_DIR ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR}/ollama)
+set(OLLAMA_INSTALL_DIR ${CMAKE_INSTALL_FULL_LIBDIR}/ollama/${OLLAMA_RUNNER_DIR})

 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY         ${OLLAMA_BUILD_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG   ${OLLAMA_BUILD_DIR})