Implement a workaround for a bug in AC_FC_PP_DEFINE.

configure.ac

@@ -252,6 +252,10 @@ AC_LANG_PUSH([Fortran])
 ACX_FC_PP_SRCEXT([f90])
 AC_LANG_POP([Fortran])
 AC_FC_PP_DEFINE
+# Due to a bug in AC_FC_PP_DEFINE the value of ac_fc_srcext is set to 'F',
+# which means that all the following tests are run not for *.f90, which we
+# need, but for *.F files. Therefore, we restore the value ourselves:
+ac_fc_srcext=f90
 
 # Set FCDEFS and adjust -D flag if needed.
 FCDEFS="$DEFS -DHAVE_FCONFIG_H"