Make switch back to assumed size arrays a configure option

Using Fortran assumed size arrays makes debugging a lot harder,
thus as default these are not used in ELPA.
However, it might be, that some compilers make unwanted copies
if calling subroutines with array slices. Then switching back
to assumed size arrays might create a performance gain

configure.ac

@@ -842,9 +842,17 @@ DX_MAN_FEATURE(ON)
 DX_HTML_FEATURE(ON)
 DX_INIT_DOXYGEN([ELPA], [Doxyfile], [docs])
 
-DESPERATELY_WANT_ASSUMED_SIZE=0
+DESPERATELY_WANT_ASSUMED_SIZE=no
+AC_MSG_CHECKING(whether --enable-assumed-size is specified)
+AC_ARG_ENABLE([assumed-size],
+                AS_HELP_STRING([--enable-assumded-size],
+                               [use assumed-size Fortran arrays]),
+              [],
+              [DESPERATELY_WANT_ASSUMED_SIZE=no])
+AC_MSG_RESULT([${DESPERATELY_WANT_ASSUMED_SIZE}])
+AM_CONDITIONAL([WITH_DESPERATELY_WANT_ASSUMED_SIZE],[test x"$DESPERATELY_WANT_ASSUMED_SIZE" = x"yes"])
 if test x"${DESPERATELY_WANT_ASSUMED_SIZE}" = x"yes" ; then
-  AC_DEFINE([DESPERATELY_WANT_ASSUMED_SIZE],[1],[use assumed size arrays, even if not debuggable])
+  AC_DEFINE([DESPERATELY_WANT_ASSUMED_SIZE],[1],[use assumed size Fortran arrays, even if not debuggable])
 fi
 
 AC_SUBST([WITH_MKL])