WIP: Working check for default real 8 flag

configure.ac

@@ -80,10 +80,34 @@ AC_F77_LIBRARY_LDFLAGS  # Determine -l and -L flags, append to $FLIBS
 AC_F77_WRAPPERS         # Discover how to call F77 funcs from C, set F77_FUNC macro.  If underscore, then Nek needs -DUNDERSCORE
 
 # Discover flags for Fortran preprocessing
-FC_temp="$F77"; FC="$F77"
+OLD_FC="$F77"; FC="$F77"
 AC_FC_PP_SRCEXT([f])  # Discover what flags are needed to preprocess .f files
 AC_FC_PP_DEFINE       # Discover the flag (eg, -D, -Df) is used to define PP symbols
-FC="$FC_temp"
+FC="$OLD_FC"
+
+###############################################################################
+# FIND F77 FLAG FOR DEFAULT 8-BYTE FLOATS
+###############################################################################
+
+AC_LANG_PUSH([Fortran 77])
+for flag in                             dnl
+  "-r8 -fpconstant"                     dnl Intel
+  "-r8"                                 dnl PGI, some others
+  "-fdefault-real-8 -fdefault-double-8" dnl GNU
+  "-qrealsize=8 -qdpc=e"                dnl IBM
+  "-s default64";                       dnl Cray
+do
+  OLD_FFLAGS="$FFLAGS"
+  FFLAGS="$flag"
+  AC_MSG_CHECKING([if Fortran 77 compiler accepts $flag])
+  AC_COMPILE_IFELSE( 
+    AC_LANG_PROGRAM(, [[      real foo]],),
+    AC_MSG_RESULT([yes]) && break,
+    AC_MSG_RESULT([no])
+  )
+done
+AC_LANG_POP([Fortran 77])
+FFLAGS="$OLD_FFLAGS"
 
 ###############################################################################
 # SET NEK's PREPROCESSOR MACROS