Fix build on tcc with glibc 2.35

Building ksh with the tcc (tinycc) compiler failed as of glibc
commit 5d98a7da. The NEWS file in that commit adds:

+* When _DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined,
+  PTHREAD_STACK_MIN is no longer constant and is redefined to
+  sysconf(_SC_THREAD_STACK_MIN).

The tcc build failure seeminly had nothing to do with that --
however, deleting the PTHREAD_STACK_MIN entry and its dependent
THREAD_STACK_MIN entry from conf.tab fixes the build failure.

For reasons unknown, gcc didn't have a problem with it. However,
these config identifiers aren't used anywhere in the ast codebase
(including the full ast-open-history repo) so it should be fine to
just get rid of them; ksh is not and will not be threaded.

NOTE: To build ksh with tcc, you need to build the latest tcc code
from <https://repo.or.cz/tinycc>. The tcc release packages in OS
distributions are typically too old and will not work.

Resolves: https://github.com/ksh93/ksh/issues/437
Thanks to @JohnoKing for the report.

src/lib/libast/comp/conf.tab

@@ -441,7 +441,6 @@ PRIO_IO				POSIX	PC 1 FUW
 PROC_RSRC_MGR			C	QQ 1 L
 PTHREAD_DESTRUCTOR_ITERATIONS	C	QQ 1 L
 PTHREAD_KEYS_MAX		C	QQ 1 L
-PTHREAD_STACK_MIN		C	QQ 1 L
 PTHREAD_THREADS_MAX		C	QQ 1 L
 PTRDIFF_MAX			C	XX 1 LMU	stdint.h 65535
 PTRDIFF_MIN			C	XX 1 LMU	stdint.h -65535
@@ -556,7 +555,6 @@ THREAD_PRIO_PROTECT		POSIX	SC 1 CDFUW
 THREAD_PROCESS_SHARED		POSIX	SC 1 CDFUW
 THREAD_SAFE_FUNCTIONS		POSIX	SC 1 CDFUW
 THREAD_SPORADIC_SERVER		POSIX	SC 1 FUW
-THREAD_STACK_MIN		POSIX	SC 1 LUW	PTHREAD_STACK_MIN
 THREAD_THREADS_MAX		POSIX	SC 1 LMUW	PTHREAD_THREADS_MAX 64
 TIMEOUTS			POSIX	SC 1 FUW
 TIMERS				POSIX	SC 1 CDFUW