acinclude.m4: fix krb5-config detection and usage in PHP_SETUP_KERBEROS.

When building with kerberos support (--with-kerberos), a few libraries
and flags need to be added to various parts of the build system. The
most reliable way to get those flags is through the krb5-config
program that ships with both major implementations of kerberos. The
PHP_SETUP_KERBEROS macro in acinclude.m4 attempts to detect
krb5-config, and use it.

However, there's a bug in that macro. The --with-kerberos parameter
accepts a directory where the kerberos libraries can be found. When a
directory is given, it is stored in the PHP_KERBEROS variable. The
following test,

  if test "$PHP_KERBEROS" = "yes" && test -x "$KRB5_CONFIG"; then

thus fails whenever a directory is passed to --with-kerberos, since it
compares a directory name against the string "yes". This causes
krb5-config to go unused, and some unreliable fallback logic is
attempted instead. One consequence of this is that the Heimdal
kerberos implementation cannot be substituted for the MIT one, at
least when a directory is passed to --with-kerberos.

This commit reverses the logic and checks for "$PHP_KERBEROS" != "no".
To confirm that this fixes the issue, one can inspect the "-l" library
flags that get appended to the command-line. On a machine with Heimdal
and the unmodified acinclude.m4, running

  ./configure --with-openssl --with-kerberos=/usr

will log (for example) to config.log,

  configure:18082: checking for krb5-config
  configure:18101: found /usr/bin/krb5-config
  configure:18114: result: /usr/bin/krb5-config
  configure:18450: checking for RAND_egd
  configure:18450: cc ... conftest.c ... -lgssapi_krb5 -lkrb5 ...

which are the library names for the MIT implementation. After patching
acinclude.m4 to negate the logic, the same command on the same machine
outputs (to config.log):

  configure:18450: cc ... conftest.c -lgssapi -lheimntlm ...

These are the correct library names for the Heimdal implementation.

PHP-Bug: 73214

Author: orlitzky

acinclude.m4

@@ -2264,7 +2264,7 @@ AC_DEFUN([PHP_SETUP_KERBEROS],[
   fi
 
   dnl If krb5-config is found try using it
-  if test "$PHP_KERBEROS" = "yes" && test -x "$KRB5_CONFIG"; then
+  if test "$PHP_KERBEROS" != "no" && test -x "$KRB5_CONFIG"; then
     KERBEROS_LIBS=`$KRB5_CONFIG --libs gssapi`
     KERBEROS_CFLAGS=`$KRB5_CONFIG --cflags gssapi`