commit e67d9c6e3139ec545cc90935e0d2cf7b97f1b709
author Brad King <brad.king@kitware.com> Mon Jul 27 09:07:42 2020 -0400
committer Brad King <brad.king@kitware.com> Mon Jul 27 10:53:43 2020 -0400
tree e78afd488d70fb1d96bb210a03146a0efe9c6999
parent 7c99b7d5dafdbff264b5b8d7e96ec81f0c4892cd

Compilers: Ignore -print-sysroot prefix when it is '/'

Since commit 8cc384f629 (Compilers: Add paths from -print-sysroot to
system prefix path, 2020-03-25, v3.18.0-rc1~337^2) we prepend the
compiler's sysroot to `CMAKE_SYSTEM_PREFIX_PATH`.  This does not
make sense when the prefix is just `/`, such as on Ubuntu 16.04's
system compiler.

Fixes: #21019

Modules/CMakeDetermineCompiler.cmake

diff --git a/Modules/CMakeDetermineCompiler.cmake b/Modules/CMakeDetermineCompiler.cmake
index 7cd7c47..cb1ab1d 100644
--- a/Modules/CMakeDetermineCompiler.cmake
+++ b/Modules/CMakeDetermineCompiler.cmake
@@ -140,7 +140,9 @@
       OUTPUT_VARIABLE _cmake_sysroot_run_out
       ERROR_VARIABLE _cmake_sysroot_run_err)
 
-    if(_cmake_sysroot_run_out AND NOT _cmake_sysroot_run_err AND IS_DIRECTORY "${_cmake_sysroot_run_out}/usr")
+    if(_cmake_sysroot_run_out AND NOT _cmake_sysroot_run_err
+        AND NOT _cmake_sysroot_run_out STREQUAL "/"
+        AND IS_DIRECTORY "${_cmake_sysroot_run_out}/usr")
       file(TO_CMAKE_PATH "${_cmake_sysroot_run_out}/usr" _cmake_sysroot_run_out_usr)
       set(CMAKE_${lang}_COMPILER_SYSROOT "${_cmake_sysroot_run_out_usr}" PARENT_SCOPE)
     else()