For a long time I've been unable to build fpc docs from svn. I finally had a good look at it and came up with an ugly workaround.

Start with a freshly-built svn fpc (with no msg2inc) or rm .../fpc/compiler/msg2inc. In .../fpcdocs run "make html". It fails with:

   cp styles/listings.sty styles/lstdoc.sty styles/lstlang1.sty styles/lstlang2.sty styles/lstlang3.sty styles/lstmisc.sty styles/lstpatch.sty styles/syntax.sty .
   make -C ../fpc/compiler msg2inc
   make[1]: Entering directory `/home/bobb/freepascal/svn/fpc/compiler'
   /usr/bin/ppc386 -Fui386 -Fusystems -Fu../rtl/units/i386-linux -Fii386 -FE. -FUi386/units/i386-linux  -di386 -dGDB -dBROWSERLOG -Fux86 -FE. utils/msg2inc.pp
   Free Pascal Compiler version 2.0.1 [2005/10/29] for i386
   Copyright (c) 1993-2005 by Florian Klaempfl
   Target OS: Linux for i386
   Compiling utils/msg2inc.pp
   msg2inc.pp(64,3) Note: Local variable "code" is assigned but never used
   Linking ./msg2inc
   815 Lines compiled, 0.4 sec
   make[1]: Leaving directory `/home/bobb/freepascal/svn/fpc/compiler'
   ../fpc/compiler/utils/msg2inc -TE ../fpc/compiler/msg/errore.msg messages.inc
   make: ../fpc/compiler/utils/msg2inc: Command not found
   make: *** [messages.inc] Error 127

Find msg2inc:
asterix:svn> find fpc -name msg2inc
fpc/compiler/msg2inc

But fpcdocs Makefile tried to use fpc/compiler/utils/msg2inc, and failed. So in my build script I added a line to delete "utils/" after unpacking sources:

# copy source to the build directory:
tar -C ${SVNDIR} -cO --exclude .svn fpcdocs | tar -C ${BUILDDIR%/*} -x

# fix makefile:
sed -i '/messages.inc$/s|utils/||' ${BUILDDIR}/Makefile

And it builds OK.