[CMake] Bad option -fPIE for NAG Fortran compiler

[Neil Carlson]

I'm updating an older cmake configuration system, and tried using
CMAKE_POSITION_INDEPENDENT_CODE.  While it mostly does
the right thing, I found that the flag -fPIE was being used for a source
file that gets compiled into an executable.  The NAG Fortran compiler
doesn't recognize this flag and exits with an error.

I don't know who is setting this; perhaps it is a default guess that
specific compiler files (NAG-Fortran.cmake) are supposed to override?
I also don't understand what the difference is between PIC and PIE.

The NAG compiler doesn't provide a PIE option.  But adding this
set(CMAKE_Fortran_COMPILE_OPTIONS_PIE "-PIC")
seems to work fine (and was almost certainly what was happening before.)

Alternatively, since the NAG Fortran compiler actually just generates C
code and then uses the underlying system C compiler, I've found this
also works
 set(CMAKE_Fortran_COMPILE_OPTIONS_PIE "-Wc,-fPIE")
on Linux.  But since that varies with platform, maybe the first option
is the safest.

Is the bottom of NAG-Fortran.cmake the right place to add this setting?
 (Brad?)

Thanks, Neil
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