Exclude raw Makefiles & gasnet from release tgzs

 - Only include officially supported CMake base build for end users
 - Of course this includes the "easy mode" shell script that wraps CMake
 - Exclude the src/gasnet dir from release archives because MPI is the current
   official backend. Only advanced users should try the gasnet version which
   is probably quite stale.

Author: zbeekman

.gitattributes

@@ -32,6 +32,10 @@ developer-scripts export-ignore
 codecov.yml export-ignore
 *.enc export-ignore
 .github export-ignore
+makefile export-ignore
+Makefile export-ignore
+make.inc* export-ignore
+src/gasnet export-ignore
 
 # Perform substitutions when `git export`ing these files
 .VERSION export-subst