Build: Fix --disable-threads breaking the building of shared libs.

This is broken in the releases 5.2.6 to 5.4.2. A workaround
for these releases is to pass EGREP='grep -E' as an argument
to configure in addition to --disable-threads.

The problem appeared when m4/ax_pthread.m4 was updated in
the commit 6629ed929c which
introduced the use of AC_EGREP_CPP. AC_EGREP_CPP calls
AC_REQUIRE([AC_PROG_EGREP]) to set the shell variable EGREP
but this was only executed if POSIX threads were enabled.
Libtool code also has AC_REQUIRE([AC_PROG_EGREP]) but Autoconf
omits it as AC_PROG_EGREP has already been required earlier.
Thus, if not using POSIX threads, the shell variable EGREP
would be undefined in the Libtool code in configure.

ax_pthread.m4 is fine. The bug was in configure.ac which called
AX_PTHREAD conditionally in an incorrect way. Using AS_CASE
ensures that all AC_REQUIREs get always run.

Thanks to Frank Busse for reporting the bug.
Fixes: https://github.com/tukaani-project/xz/issues/45

configure.ac

@@ -611,8 +611,8 @@ AM_PROG_CC_C_O
 AM_PROG_AS
 AC_USE_SYSTEM_EXTENSIONS
 
-case $enable_threads in
-	posix)
+AS_CASE([$enable_threads],
+	[posix], [
 		echo
 		echo "POSIX threading support:"
 		AX_PTHREAD([:]) dnl We don't need the HAVE_PTHREAD macro.
@@ -643,19 +643,19 @@ case $enable_threads in
 			`CLOCK_MONOTONIC' is declared in <time.h>.])], [],
 			[[#include <time.h>]])
 		CFLAGS=$OLD_CFLAGS
-		;;
-	win95)
+	],
+	[win95], [
 		AC_DEFINE([MYTHREAD_WIN95], [1], [Define to 1 when using
 			Windows 95 (and thus XP) compatible threads.
 			This avoids use of features that were added in
 			Windows Vista.])
-		;;
-	vista)
+	],
+	[vista], [
 		AC_DEFINE([MYTHREAD_VISTA], [1], [Define to 1 when using
 			Windows Vista compatible threads. This uses
 			features that are not available on Windows XP.])
-		;;
-esac
+	]
+)
 AM_CONDITIONAL([COND_THREADS], [test "x$enable_threads" != xno])
 
 echo