BuildSystem: Avoid == for comparison in ./configure.

ASTERISK-27709
Reported by: John Nemeth

Change-Id: I11b1ae8fd404c04066f1458f5d71f9536359d58d

configure

@@ -14105,7 +14105,7 @@ fi
 
 
 
-if test "x$JANSSON_LIB" == "x"; then
+if test "${PBX_JANSSON}" != 1; then
   as_fn_error $? "*** JSON support not found (this typically means the libjansson development package is missing)" "$LINENO" 5
 fi
 

configure.ac

@@ -659,7 +659,7 @@ AC_SUBST(UUID_LIB)
 # Find required JSON support.
 AST_EXT_LIB_CHECK([JANSSON], [jansson], [json_dumps], [jansson.h])
 
-if test "x$JANSSON_LIB" == "x"; then
+if test "${PBX_JANSSON}" != 1; then
   AC_MSG_ERROR([*** JSON support not found (this typically means the libjansson development package is missing)])
 fi