"/usr/bin/ld: cannot find -lX11" error when installing htk

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http://askubuntu.com – I am trying to use htk (HMM tool kit) for speech recognition. I follow the instruction guideline in 'readme' file. Firstly, I typed './configure' and it worked. Followings are terminal output when I typed './configure'. checking whether make sets $(MAKE)... yes checking for gawk... no checking for mawk... mawk checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using (HowTos)