cmake: check if NIX_CC exists before using it

pkgs/development/tools/build-managers/cmake/001-search-path.diff

@@ -12,7 +12,7 @@ index b9381c3d7d..5e944640b5 100644
    # CMake install location
    "${_CMAKE_INSTALL_DIR}"
    )
-@@ -47,48 +44,48 @@ endif()
+@@ -47,48 +44,50 @@ endif()
 
  # Non "standard" but common install prefixes
  list(APPEND CMAKE_SYSTEM_PREFIX_PATH
@@ -22,14 +22,16 @@ index b9381c3d7d..5e944640b5 100644
    )
 
  # List common include file locations not under the common prefixes.
-+if(IS_DIRECTORY $ENV{NIX_CC}
-+  AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc
-+  AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc-dev)
-+  file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc" _nix_cmake_libc)
-+  file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc-dev" _nix_cmake_libc_dev)
-+else()
-+  set(_nix_cmake_libc @libc_lib@)
-+  set(_nix_cmake_libc_dev @libc_dev@)
++if(DEFINED ENV{NIX_CC})
++  if(IS_DIRECTORY $ENV{NIX_CC}
++    AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc
++    AND EXISTS $ENV{NIX_CC}/nix-support/orig-libc-dev)
++    file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc" _nix_cmake_libc)
++    file(STRINGS "$ENV{NIX_CC}/nix-support/orig-libc-dev" _nix_cmake_libc_dev)
++  else()
++    set(_nix_cmake_libc @libc_lib@)
++    set(_nix_cmake_libc_dev @libc_dev@)
++  endif()
 +endif()
 +
  list(APPEND CMAKE_SYSTEM_INCLUDE_PATH