Add additional common C++ and Fortran header file extensions (#11600)

* Add additional common C++ and Fortran header file extensions

* Add .hxx extension

* Add .txx and .tcc extensions

* Add .icc extension

lib/spack/llnl/util/filesystem.py

@@ -1141,7 +1141,14 @@ def find_headers(headers, root, recursive=False):
         raise TypeError(message)
 
     # Construct the right suffix for the headers
-    suffixes = ['h', 'hpp', 'mod']
+    suffixes = [
+        # C
+        'h',
+        # C++
+        'hpp', 'hxx', 'hh', 'H', 'txx', 'tcc', 'icc',
+        # Fortran
+        'mod', 'inc',
+    ]
 
     # List of headers we are searching with suffixes
     headers = ['{0}.{1}'.format(header, suffix) for header in headers