Module files won't use CPATH by default, but language specific vars (#14749)

fixes #11555

Every path in CPATH is equivalent to a -I path to the compiler,
while every path in *_INCLUDE_PATH is equivalent to -isystem.
The latter avoids the noise due to warnings coming from 3rd party
libraries that a project depends on.

Added INCLUDE env variable (Intel Fortran, .mod files)

etc/spack/defaults/modules.yaml

@@ -30,7 +30,11 @@ modules:
     lib64:
       - LIBRARY_PATH
     include:
-      - CPATH
+      - C_INCLUDE_PATH
+      - CPLUS_INCLUDE_PATH
+      # The INCLUDE env variable specifies paths to look for
+      # .mod file for Intel Fortran compilers
+      - INCLUDE
     lib/pkgconfig:
       - PKG_CONFIG_PATH
     lib64/pkgconfig: