cmake: fix crash on CC without libc support

Without this change pkgsLLVM fails to build any packages
as compiler-rt fails early in cmake:

    CMake Error at ...-cmake-3.24.0/share/cmake-3.24/Modules/Platform/UnixPaths.cmake:53 (file):
      file STRINGS file
      "...-x86_64-unknown-linux-gnu-clang-wrapper-11.1.0/nix-support/orig-libc-dev"
      cannot be read.

It's a regression caused by 871cf9f "cmake: detect libc location
at runtime NixOS#181431" where we started using `orig-libc-dev` as a libc pointer.

During pkgsLLVM pootstrap first compiler has no libc support yet.

The change skips runtime detection if there are no libc signs.

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 +44,46 @@ endif()
+@@ -47,48 +44,48 @@ endif()
 
  # Non "standard" but common install prefixes
  list(APPEND CMAKE_SYSTEM_PREFIX_PATH
@@ -22,7 +22,9 @@ index b9381c3d7d..5e944640b5 100644
    )
 
  # List common include file locations not under the common prefixes.
-+if(IS_DIRECTORY $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()