Known conditionals:

* MONO_FEATURE_WEB_STACK: when we have the full web stack.

* MONO_FEATURE_NEW_TLS: we are using the new TLS implementation.

* MONO_FEATURE_LOGGING: whether we want the internal logging API.

* MONO_NOT_SUPPORTED: not supported in Mono.

* MONO_NOT_IMPLEMENTED: not yet implemented in Mono.

Other defines which are used when building the BCL and tests:

* MONO_INSIDE_SYSTEM: we're building System.dll.

* MONO_INSIDE_MONO_SECURITY: we're building Mono.Security.dll.

Extern aliases:

* MONO_SECURITY_ALIAS: we're using Mono.Security from the "MonoSecurity" extern alias.

* MONO_X509_ALIAS: we're using X509CertificateCollection from the "PrebuiltSystem" extern alias.

