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Follow the recommendations from the Autoconf documentation on the usage of the Fortran preprocessor flag.

The preprocessor flag "must appear immediately before the source-code file name
when compiling" (see the description of AC_FC_PP_SRCEXT in Autoconf manual).
Moreover, the flag should not appear in the linking command because it might
confuse the compiler, which is, for example, the case for Cray compiler when
the preprocessor flag is equal to '-eZ' (but not to 'eT').

* Removed the output variable FCFDEFS, which was generated based on the value
  of the variable DEFS, because the latter always equals to '-DHAVE_CONFIG' and
  this macro defenition is not needed for the Fortran source files.
* Removed a redundant declaration of FCFLAGS as an output variable: Autoconf
  does this automatically.
* Removed the output variable FCLDFLAGS, which was always empty.
* Removed the custom postprocessing of makefiles with sed, which was supposed
  to modify the compilation and linking commands to account for FCDEFS and
  FCLDFLAGS, which either didn't work (some makefiles have custom declarations
  for FCCOMPILE that do no match the sed pattern, some other do not compile
  anything at all) or resulted in linking without FCFLAGS (the variable
  FCFLAGS was replaced with the empty variable FCLDFLAGS), which might fail
  the linking, e.g. when the flag enabling OpenMP support is provided by the
  user in FCFLAGS.

Create directories before generating Fortran dependencies to avoid warning messages from the preprocessor when building out of source.

* Previously this had to be part of the user-specified flags. This
  requirement was not easily discoverable. Now PPM should have less
  problems with PGI compilers.

* Unfortunately it is not clear which compiler version of pgfortran
  re-introduced the failure.

* Declare unused parameter and inspect return value of call.

* Previous releases printed something like

pgfortran 14.7-0 64-bit target on x86-64 Linux -tp sandybridge
The Portland Group - PGI Compilers and Tools
Copyright (c) 2014, NVIDIA CORPORATION.  All rights reserved.

* Newer ones omit the string detected by older libtool versions, printing
  a text like

pgfortran 18.10-0 64-bit target on x86-64 Linux -tp haswell
PGI Compilers and Tools
Copyright (c) 2018, NVIDIA CORPORATION.  All rights reserved.

* The PGI compiler nowadays recognizes enough options to be confused with
  the NAG and Sun compilers, but is perfectly able to honour the simpler -E
  option.