I found a Solaris system.

$ uname -a
SunOS <<REDACTED>> 5.10 Generic_148888-05 sun4u sparc SUNW,Sun-Blade-1000

On it I built and installed a thread-enabled Tcl 8.4.9 from sources.
I used the "Forte Developer 7 C 5.4 compiler.

The small program you submitted doesn't produce a working executable
of course.  Here's the modified source I used:

$ cat hang.c
#include <tcl.h>
int main()
{
 int x=0;
 Tcl_Interp* a;
 a =  Tcl_CreateInterp();
 if (a == NULL)
 printf("Interpreter creation Failed \n");
 Tcl_Init(a);
 Tcl_Eval(a, "puts \"Tcl [info patchlevel] thread:$tcl_platform(threaded)\"");
 Tcl_DeleteInterp(a);
 printf("DeleteInterpreter \n");
}

$ cc -ltcl8.4 hang.c

$ a.out
Tcl 8.4.9 thread:1
DeleteInterpreter

It does not hang.  It returns quickly, with no delay that I can
perceive.

In short, I cannot reproduce the problem you report from the
description you've offered.  dkf may have different thoughts,
but I'm not interested in seeing logs, I'm interested in the
ability to demonstrate the problem.

If I cannot demonstrate the problem, I see no hope that I can
offer any help.