configure: compare which's result against "x", not "xno"

This was apparently copy-pasted from the AC_PATH_PROG checks above.
`which` doesn't return a "no" result if it fails, unlike AC_PATH_PROG.

This typo would make the configure phase pass even if there were no
gapi tools installed.

configure.ac

@@ -107,7 +107,7 @@ AC_PATH_PROG(GAPI_PARSER, gapi2-parser, no)
 if test "x$GAPI_PARSER" = "xno"; then
 	AC_MSG_CHECKING(for gapi2-parser.exe)
 	GAPI_PARSER=`$WHICH gapi2-parser.exe`
-	if test "x$GAPI_PARSER" = "xno" ; then
+	if test "x$GAPI_PARSER" = "x" ; then
 		AC_MSG_ERROR(['gapi2-parser'/'gapi2-parser.exe' not found.])
 	fi
 	AC_MSG_RESULT($GAPI_PARSER)
@@ -119,7 +119,7 @@ AC_PATH_PROG(GAPI_FIXUP, gapi2-fixup, no)
 if test "x$GAPI_FIXUP" = "xno"; then
 	AC_MSG_CHECKING(for gapi2-fixup.exe)
 	GAPI_FIXUP=`$WHICH gapi2-fixup.exe`
-	if test "x$GAPI_FIXUP" = "xno" ; then
+	if test "x$GAPI_FIXUP" = "x" ; then
 		AC_MSG_ERROR(['gapi2-fixup'/'gapi2-fixup.exe' not found.])
 	fi
 	AC_MSG_RESULT($GAPI_FIXUP)
@@ -131,7 +131,7 @@ AC_PATH_PROG(GAPI_CODEGEN, gapi2-codegen, no)
 if test "x$GAPI_CODEGEN" = "xno"; then
 	AC_MSG_CHECKING(for gapi2_codegen.exe)
 	GAPI_CODEGEN=`$WHICH gapi2_codegen.exe`
-	if test "x$GAPI_CODEGEN" = "xno" ; then
+	if test "x$GAPI_CODEGEN" = "x" ; then
 		AC_MSG_ERROR(['gapi2-codegen'/'gapi2_codegen.exe' not found.])
 	fi
 	AC_MSG_RESULT([$GAPI_CODEGEN])