Instruction for old systems:

I have tested build with gcc-3.4.6, gcc-4.1.2, gcc-4.2.4 and gcc-4.3.5.
Before build copy or symlink the p subdirectry of gpc into gcc-x.y.z/gcc
subdirectory of gcc tree (that is you should have directory called
gcc-x.y.z/gcc/p).  To build with gcc-3.4.6 (or earlier) apply backend
patch.   That is in gcc-3.4.6/gcc do

patch -p1 < p/diff/gcc-3.4.4.diff

Next, create build directory.  In the build directory configure inculuding
Pascal in list of languages to build like

../gcc-3.4.6/configure  --enable-languages=c,pascal --disable-multilib

Note: GPC does not support multilibs, the '--disable-multilib' provents
build failure due to this.

To build with gcc-4.y copy the libgpc subdirectory into toplevel gcc
directory (that is into gcc-4.y.z) and apply the backend patch
from _toplevel_ directory (that is inside gcc-4.x.y directory) using
command:

patch -p1 < gcc/p/gcc-4.y.z.diff

Then configure and build.  Using gcc-4.3.5 you need to give 'configure' 
the '--disable-mapped-location' argument.

Newer Linux systems:

On newer Debian systems (and possibly other Linux distributions) there
is changed layout of include files and library files: several include files
are in platform specific subdirectories like '/usr/include/x86_64-linux-gnu'
and its subdirectories.  Similarly libraries are in
'/usr/lib/x86_64-linux-gnu'.  Unpached gcc-3.4.6 can not find system include
files and as result build fails.  If problem with include files is
worked around, then gcc-3.4.6 can not find essential library files and
again build will files.   To work around both problems do the
following in the toplvel gcc-3.4.6 directory:

sed 's,@TARGET_TRIPLET@,x86_64-linux-gnu,g' gcc/p/multilib-3.4.diff \
  | patch -p1

Note: you need here "canonical name" of your platform, so if it is
different replace 'x86_64-linux-gnu' by name of your platform.  You
can verify if platform name is correct by checking for presence of
directories like '/usr/include/x86_64-linux-gnu' and
'/usr/lib/x86_64-linux-gnu'.

Note: Do the above if you build native compiler and your Linux uses
layout as above.  System using old layout do not need this patch.
In particular cross compilers seem to continue with old layout.

In recent Linux systems declaration of 'ucontext' changed in incompatible
way.  If your system uses new declaration then you need to apply extra
patch in toplevel gcc-3.4.6 directory:

patch -p1 < gcc/p/ucontext.diff

After applying the two patches (if needed) proceed as on older systems.

If you apply the 'multilib-3.4.diff' patch on platform that do not
need it, it may cause build failure.  If you apply 'ucontext.diff'
on platform that do not need it, you almost surely will get build
failure.  So, you need to find out features of you system and
apply only relevant patches.  Idealy this would be automated,
but ATM this must be done by hand.

Known problems:

1) with gcc-4.y.z testsuite will fail if run immediately after build.
Either install before running testsuite or copy 'libgpc.a' from libgpc
build directory to gcc subdirectory and copy rts-confing.inc from libgpc
build directory to gcc/p/rts subdirectory (you need to create rts
subdirectiry)
2) I did not try to build shared libgpc -- it is probably broken.
You would need to fix Makefiles to build it.
3) -Wparenthesis does not work with gcc-4.3.z
4) Utilities (binobj and gpidump) are not build with gcc-4.y.z