exec-prefix is one of the standard variables in autoconf-generated (and possibly other?) configure scripts and makefiles. It is a prefix of the directory name into which platform/architecture-dependent portions of the package (e.g. executables and dynamic libraries) should be installed. In some cases it defaults to the value set for the prefix variable, in other cases to a subdirectory of $prefix, cf. Generating a generic platform name.
As TEA uses autoconf, many Tcl extensions and packages have an exec-prefix value that was set at the time of build-configuring the package in question. Q: When does this matter at the script level? Is it merely providing defaults for init.tcl, or are there other issues at hand?