Move JSON_CFLAGS before LIBPROXY_CFLAGS
Depending on build options, libproxy-1.0.pc depends indirectly on json-c.pc:
libproxy-1.0 -> gio-2.0 -> mount -> libcryptsetup -> json-c
This causes "pkg-config --cflags libproxy-1.0" to emit "-I/usr/include/json-c".
json-c installs a "json.h" file that conflicts with the one provided by json-parser. If json-c comes before json-parser on the compiler command, we get a build failure:
openconnect-internal.h:1654:59: error: unknown type name 'json_value'