configure: add --without-mdb flag (fix issue #6439)

[cgalibern]

We need a way to disable node_use_mdb on non Illumos derivated, because of missing libproc.h.

Example:
./configure --dest-cpu=x64 --dest-os=solaris --prefix=/opt/node --without-mdb
creating ./icu_config.gypi
{ 'target_defaults': { 'cflags': [],
'default_configuration': 'Release',
'defines': [],
'include_dirs': [],
'libraries': []},
'variables': { 'clang': 0,
'gcc_version': 48,
'host_arch': 'ia32',
'icu_small': 'false',
'node_install_npm': 'true',
'node_prefix': '/opt/node',
'node_shared_cares': 'false',
'node_shared_http_parser': 'false',
'node_shared_libuv': 'false',
'node_shared_openssl': 'false',
'node_shared_v8': 'false',
'node_shared_zlib': 'false',
'node_tag': '',
'node_use_dtrace': 'true',
'node_use_etw': 'false',
'node_use_mdb': 'false',
'node_use_openssl': 'true',
'node_use_perfctr': 'false',
'openssl_no_asm': 0,
'python': '/usr/bin/python',
'target_arch': 'x64',
'uv_library': 'static_library',
'uv_parent_path': '/deps/uv/',
'uv_use_dtrace': 'true',
'v8_enable_gdbjit': 0,
'v8_enable_i18n_support': 0,
'v8_no_strict_aliasing': 1,
'v8_optimized_debug': 0,
'v8_random_seed': 0,
'v8_use_snapshot': 'true',
'want_separate_host_toolset': 1}}
creating ./config.gypi
creating ./config.mk