SETGPC.BAT does not allow me to compile C using gpc:
           gpc tc.c -o tc
     As installed by dev_gnu_pascal-1.9.4.13.exe, SETGPC.BAT:
          set PATH=C:\prg\GPC\bin;%PATH%
          set C_INCLUDE_PATH=C:\prg\GPC\include
          set GCC_BASE=3.4.5
          set GPC_EXEC_PREFIX=C:\prg\GPC\libexec\gcc\mingw32
          set LIBRARY_PATH=C:\prg\GPC\lib;C:\prg\GPC\lib\gcc\mingw32\%GCC_BASE%
          set GPC_UNIT_PATH=C:\prg\GPC\lib\gcc\mingw32\%GCC_BASE%\units;C:\prg\GPC\units;C:\prg\GPC\units\winapi;C:\prg\GPC\units\objects;C:\prg\GPC\units\sysutils;C:\prg\GPC\units\objmingw
     I have been able to compile and link Pascal and C (one each) by
only changing PATH.
     Separately, I succeeded by appending 
c:\prg\gpc\lib\gcc\mingw32\3.4.5\include to C_INCLUDE_PATH.
(After running SETGPC.BAT)
     I don't know what to say is wrong.  The lack of a trailing slash on
GPC_EXEC_PREFIX results in a horrible mess in one of the attempted
directories
(C:/prg/GPC/libexec/gcc/mingw32mingw32/3.4.5/../../../../include/stddef.h)
but appending a slash doesn't solve the real problem, because "libexec"
is the wrong path to get to an include directory.
     Does GPC have different rules for the EXEC_PREFIX than GCC, so the
lack of a trailing slash is not a mistake for GPC_EXEC_PREFIX?
     Does the setting of all of the variables other than PATH date back
to a time before GPC did such an elaborate job of fabricating
directory names?  Or before GPC had the target and version wired in?
     Is it likely that all I need is to prepend GPC/bin to PATH?
Or do you think I will need some of the other environment variables
when I get around to trying to compile more substantial programs?