Add a variable to specify language-wide system include directories

Create a `CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES` variable to specify
system include directories for for `<LANG>` compiler command lines.
This plays a role for include directories as the existing
`CMAKE_<LANG>_STANDARD_LIBRARIES` variable does for link libraries.

1 files changed, 5 insertions, 0 deletions

diff --git a/Tests/IncludeDirectories/StandardIncludeDirectories/main.c b/Tests/IncludeDirectories/StandardIncludeDirectories/main.c
new file mode 100644
index 0000000..edfe9ce
--- /dev/null
+++ b/Tests/IncludeDirectories/StandardIncludeDirectories/main.c
@@ -0,0 +1,5 @@
+#include "StdIncDir.h"
+int main()
+{
+  return 0;
+}