package/leveldb: fix build for gcc < 5.x

package/leveldb: fix build for gcc < 5.x

Without defining LEVELDB_HAS_PORT_CONFIG_H port/port_config.h will not
be included:
https://github.com/google/leveldb/blob/master/port/port_stdcxx.h#L8

which breaks endian detection. Add the define to our memenv patch to fix
building with gcc < 5.
__has_include was added in gcc5: https://gcc.gnu.org/gcc-5/changes.html

Fixes:
http://autobuild.buildroot.net/results/4ac/4ac8fbed79307326086ab294eba9e68cf208e6ac/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>