removes noexternal option and adds remark about compilation flag (noe…

…xternal not recognized by buildbot version ifort 13.0)

flags.guess

@@ -90,8 +90,9 @@ case $my_FC in
         # I/O throughput lingers at 2.5 MB/s, with it it can increase to ~44 MB/s
         # However it does not make much of a difference on NFS mounted volumes or with SFS 3.1.1 / Lustre 1.6.7.1
         #
+        # warnings about external function calls can be suppressed by "-warn all,noexternal" for version > 2018
         # optimization report: "-vec-report0" is old and will be replaced by "-qopt-report0 -qopt-report-phase=vec" for v >=15.0
-        DEF_FFLAGS="-xHost -fpe0 -ftz -assume buffered_io -assume byterecl -align sequence -std03 -diag-disable 6477 -implicitnone -gen-interfaces -warn all,noexternal" # -mcmodel=medium -shared-intel
+        DEF_FFLAGS="-xHost -fpe0 -ftz -assume buffered_io -assume byterecl -align sequence -std03 -diag-disable 6477 -implicitnone -gen-interfaces -warn all" # -mcmodel=medium -shared-intel
         OPT_FFLAGS="-O3 -check nobounds"
         DEBUG_FFLAGS="-check all -debug -g -O0 -fp-stack-check -traceback -ftrapuv"
         # option "-openmp" is soon deprecated and replaced by "-qopenmp" for versions > 17.x