Use automake's nobase_nodist_include_HEADERS variable for poly2tri headers

We don't want to distribute the modified poly2tri headers that we
create during the build process, but we _do_ want to install them
(with the proper relative directory structure), so that libmesh users
can include them in their own codes.

Author: jwpeterson

contrib/poly2tri/modified/Makefile.am

@@ -27,7 +27,22 @@ poly2tri_headers = poly2tri/poly2tri.h \
                    poly2tri/sweep/sweep_context.h \
                    poly2tri/sweep/sweep.h
 
-nodist_include_HEADERS = $(poly2tri_headers)
+# includedir is an automake variable, and we are just using its default value here
+includedir             = $(prefix)/include
+
+# nobase_include_HEADERS are headers which should be _installed_ while maintaining relative paths.
+# nodist_include_HEADERS are headers which should _not_ be included as part of the source distribution.
+#
+# In our case, we _do_ want to install the modified poly2tri headers
+# that we create during the build process, but we don't want to
+# distribute them.  If you separately specify the same set of headers
+# as both nobase_ and nodist_, this apparently creates a conflict for
+# automake, and it ends up including the files in the distribution
+# anyway. But, according to Google AI: "If you need both
+# behaviors--keeping the subdirectory structure AND not
+# distributing--the correct variable name is
+# nobase_nodist_include_HEADERS. The nobase_ prefix must come first."
+nobase_nodist_include_HEADERS = $(poly2tri_headers)
 
 BUILT_SOURCES = $(poly2tri_headers)