package/libmdbx: fix build with glibc < 2.12

package/libmdbx: fix build with glibc < 2.12

Set CMAKE_BUILD_TYPE to Release to avoid the following build failure
with glibc < 2.12:

/home/buildroot/autobuild/run/instance-1/output-1/build/libmdbx-0.9.2/mdbx.c:487:5: error: #warning "libmdbx was only tested with GLIBC >= 2.12." [-Werror=cpp]
 #   warning "libmdbx was only tested with GLIBC >= 2.12."
     ^~~~~~~
cc1: all warnings being treated as errors

Fixes:
 - http://autobuild.buildroot.org/results/1a60b2c3d2f276f99a22da48e8e16fcf5744eba0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Acked-by: Leonid Yuriev <leo@yuriev.ru>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>