Packages not found, X11R7.1 build terminates

[J. Hart]

I am trying to build X11R7.1 using the following procedure :
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wn X11R7.1
topdir=/home/jhart/linuxfrmsrc
cd $topdir
rm -fr tarballs modular build.log
mkdir tarballs
pushd tarballs
SVPATH=$PATH
PATH=$topdir/modular/bin:$PATH
cp -r $topdir/archives/xorg_X11R7.1/* .
cp ../archives/build-from-tarballs.sh .
chmod +x build-from-tarballs.sh
chmod -R +w *
export PKG_CONFIG_PATH=$topdir/modular/lib/pkgconfig
./build-from-tarballs.sh $topdir/modular 2>&1 | tee $topdir/build.log
unset PKG_CONFIG_PATH
PATH=$SVPATH
unset SVPATH
popd
rm -fr tarballs build.log

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I get the following error which terminates the build :
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checking for XPROTO... yes
checking for X11... configure: error: Package requirements (xextproto 
xtrans xau xcmiscproto bigreqsproto) were not met:

Package xextproto was not found in the pkg-config search path.
Perhaps you should add the directory containing `xextproto.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xextproto' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables X11_CFLAGS
and X11_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
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If this is looking for a ".pc" file related to the xextproto package, 
should this not have been included or generated by the build process ?

Does anyone know what is going wrong here ?

I tried setting the following instead of allowing pkg-config to run:
      export X11_CFLAGS=-I$topdir/modular/include
      export X11_LIBS=-L$topdir/modular/lib
      export XKBPROTO_CFLAGS=-I$topdir/modular/include
      export XKBPROTO_LIBS=-L$topdir/modular/lib

If I do this, the build eventually terminates due to a missing Xtrans.h.

Regards and Thanks in advance.