1.7.6 configure bugs

'configure' in git 1.7.6 does not check for the presence of zlib.h nor
tclsh. Compilation will fail without zlib.h and converting the languages
will fail without tclsh

make[1]: Entering directory `/home/user/tmp/git-1.7.6/git-gui'
tclsh po/po2msg.sh --statistics --tcl -l de -d po/ po/de.po
make[1]: tclsh: Command not found

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