Improve test shell builtin invocation portability

POSIX 2008 obsoletes the -a and -o operands (according to the autoconf
documentation).

acinclude.m4

@@ -605,7 +605,7 @@ for be in ${BACKENDS}; do
 
     gphoto2)
     if test "${HAVE_GPHOTO2}" != "true" \
-      -o "${sane_cv_use_libjpeg}" != "yes"; then
+      || test "${sane_cv_use_libjpeg}" != "yes"; then
       echo "*** $be backend requires gphoto2 and JPEG libraries - $DISABLE_MSG"
       backend_supported="no"
     fi

configure.ac

@@ -180,7 +180,7 @@ else
 	fi
 fi
 
-if test "$with_snmp" = "yes" -a "${LIBSNMP_EXISTS}x" = "yesx"; then
+if test "$with_snmp" = "yes" && test "${LIBSNMP_EXISTS}x" = "yesx"; then
 	AC_SUBST(SNMP_LIBS)
 	AC_SUBST(SNMP_CFLAGS)
 	AC_DEFINE(HAVE_LIBSNMP, 1, [Define to 1 if you have the net-snmp library.])