[compiler-rt] [compiler-rt] Make add_custom_libcxx() resilient to DESTDIR being set (PR #115525)

[Björn Pettersson via llvm-commits]

================
@@ -713,6 +713,9 @@ macro(add_custom_libcxx name prefix)
     USES_TERMINAL_INSTALL 1
     LIST_SEPARATOR |
     EXCLUDE_FROM_ALL TRUE
+    # Ensure that DESTDIR=... set in the out environment does not affect this
+    # target (we always need to install to the build directory).
+    INSTALL_COMMAND env DESTDIR= ${CMAKE_COMMAND} --build ${prefix}/build --target install
     INSTALL_BYPRODUCTS "${prefix}/lib/libc++.a" "${prefix}/lib/libc++abi.a"
----------------
bjope wrote:

With the INSTALL_COMMAND fix we now get this:
```
[2091/2122] Performing install step for 'libcxx_fuzzer_x86_64'
ninja: error: unknown target 'INSTALL_BYPRODUCTS'
FAILED: compiler-rt/lib/fuzzer/libcxx_fuzzer_x86_64/src/libcxx_fuzzer_x86_64-stamp/libcxx_fuzzer_x86_64-install 
```


https://github.com/llvm/llvm-project/pull/115525