LinuxQuestions.org - /usr/bin/ld cannot find -lXext

While gcc compiling a complex program (AmberTools of Amber suite, a molecular dynamics package) on amd64:

./configure_at gcc
make -f Makefile_at

After a lot of compiled packages, I am getting the error

******************
make[3]: Entering directory `/usr/local/amber10/src/leap/src/leap'
gcc -o teLeap basics.o sysdepend.o stringExtra.o varArray.o getline.o avl.o pdb_format.o pdb_read.o pdb_sprntf.o pdb_sscanf.o pdb_write.o vector.o zMatrix.o sort.o bag.o hash.o dictionary.o database.o nVector.o ring.o matrix.o fortran.o displayer.o assoc.o atom.o byteArray.o collection.o container.o internal.o list.o loop.o molecule.o oDouble.o oInteger.o oString.o objekt.o parmSet.o residue.o unit.o unitio.o tripos.o graphUtil.o select.o amber.o build.o elements.o library.o chirality.o minimizer.o model.o parmLib.o pdbFile.o mol2File.o tools.o variables.o parser.o help.o helptext.o octree.o commands.o mathop.o block.o restraint.o hybrid.o tLeap.o -lm
gcc -o xaLeap basics.o sysdepend.o stringExtra.o varArray.o getline.o avl.o pdb_format.o pdb_read.o pdb_sprntf.o pdb_sscanf.o pdb_write.o vector.o zMatrix.o sort.o bag.o hash.o dictionary.o database.o nVector.o ring.o matrix.o fortran.o displayer.o assoc.o atom.o byteArray.o collection.o container.o internal.o list.o loop.o molecule.o oDouble.o oInteger.o oString.o objekt.o parmSet.o residue.o unit.o unitio.o tripos.o graphUtil.o select.o amber.o build.o elements.o library.o chirality.o minimizer.o model.o parmLib.o pdbFile.o mol2File.o tools.o variables.o parser.o help.o helptext.o octree.o commands.o mathop.o block.o restraint.o hybrid.o xTank.o xAction.o x3d.o xBasics.o xaLeapc.o xaUnitEditor.o xaTable.o xaAtomTable.o XrawRegistr.o xaCommand.o xaTools.o xaAtomParmTable.o xaBondParmTable.o xaAngleParmTable.o xaParmEditor.o xaTorsionParmTable.o xaImproperParmTable.o xaHBondParmTable.o ../Xraw/libXaw.a ../Wc/libWcLeap.a ../Xpm/libXpm.a ../Xmu/libXmu.a -L/usr/X11R6/lib64 -L/usr/X11R6/lib -lXt -lXext -lSM -lICE -lX11 -lXau -lXdmcp -lm -lpthread
/usr/bin/ld: cannot find -lXext
collect2: ld returned 1 exit status
make[3]: *** [xaLeap] Error 1
make[3]: Leaving directory `/usr/local/amber10/src/leap/src/leap'
make[2]: *** [install_xleap] Error 2
make[2]: Leaving directory `/usr/local/amber10/src/leap'
make[1]: *** [install] Error 2
make[1]: Leaving directory `/usr/local/amber10/src/leap'
make: *** [install] Error 2
tya64:/usr/local/amber10/src#
****************

Info:

francesco@tya64:~$ apt-file search libXext
ia32-libs: /emul/ia32-linux/usr/lib/libXext.so
ia32-libs: /emul/ia32-linux/usr/lib/libXext.so.6
ia32-libs: /emul/ia32-linux/usr/lib/libXext.so.6.4.0
libxext-dev: /usr/lib/libXext.a
libxext-dev: /usr/lib/libXext.so
libxext6: /usr/lib/libXext.so.6
libxext6: /usr/lib/libXext.so.6.4.0
libxext6-dbg: /usr/lib/debug/usr/lib/libXext.so.6.4.0
lsb-build-base3: /usr/lib/lsb3/libXext.so
francesco@tya64:~$

tya64:/home/francesco# ldconfig -v | grep "libXext"
libXext.so.6 -> libXext.so.6.4.0
libXext.so.6 -> libXext.so.6.4.0
tya64:/home/francesco#

I am confused how to turnaround the problem. Thanks for help.