Package x11 not found in pkg-config search path

[J. Hart]

I am trying to build X11R7.1 using the following procedure :
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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
./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 XEXT... configure: error: Package requirements (xproto x11 
xextproto xau) were not met:

Package x11 was not found in the pkg-config search path.
Perhaps you should add the directory containing `x11.pc'
to the PKG_CONFIG_PATH environment variable
No package 'x11' 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 XEXT_CFLAGS
and XEXT_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
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Does anyone know what is going wrong here ?

Regards and Thanks in advance.