Do not exclude include directories made implicit by CPATH

Entries of the `CPATH` environment variable are implicitly searched as
include directories by some C/C++ compilers.  Since commit 5990ecb741
(Compute implicit include directories from compiler output, 2018-12-07,
v3.14.0-rc1~108^2) these entries are detected by CMake and included in
the `CMAKE_{C,CXX}_IMPLICIT_INCLUDE_DIRECTORIES` variables.

However, we should not exclude them from explicit specification via `-I`
or particularly `-isystem` because they are meant as user-specified
include directories that can be re-ordered without breaking compiler
builtin headers.  In particular, we need explicit requests via
`include_directories` with the `SYSTEM` option to result in `-isystem`
so that third-party headers do not produce warnings.

Co-Author: Ben Boeckel <ben.boeckel@kitware.com>
Fixes: #19291

1 files changed, 15 insertions, 0 deletions

diff --git a/Tests/IncludeDirectoriesCPATH/viacpath/systemlib.h b/Tests/IncludeDirectoriesCPATH/viacpath/systemlib.h
new file mode 100644
index 0000000..1aaafa9
--- /dev/null
+++ b/Tests/IncludeDirectoriesCPATH/viacpath/systemlib.h
@@ -0,0 +1,15 @@
+#ifndef SYSTEMLIB_H
+#define SYSTEMLIB_H
+
+int systemlib()
+{
+  return 0;
+}
+
+int unusedFunc()
+{
+  int unused;
+  return systemlib();
+}
+
+#endif