llvmPackages_git.libcxxabi: fix cycles that arise when stdenv is th…

…e LLVM stdenv

Port of 8db72e3
("llvmPackages_15.libcxxabi: fix cycles that arise when `stdenv` is the
LLVM stdenv").

pkgs/development/compilers/llvm/git/libcxx/default.nix

@@ -74,7 +74,13 @@ stdenv.mkDerivation rec {
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
-    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF";
+    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF"
+    # If we're only building the headers we don't actually *need* a functioning
+    # C/C++ compiler:
+    ++ lib.optionals (headersOnly) [
+      "-DCMAKE_C_COMPILER_WORKS=ON"
+      "-DCMAKE_CXX_COMPILER_WORKS=ON"
+    ];
 
   ninjaFlags = lib.optional headersOnly "generate-cxx-headers";
   installTargets = lib.optional headersOnly "install-cxx-headers";