There is no concept of compiler warnings, specifically, however, specifying compilers enable different kinds of support for different errors and warnings when compiling.
--compiler=nexfuse
Specifies that warnings for the
NexFUSE binary format should be
enabled.
--compiler=openlud
Specifies that warnings for the
OpenLUD binary format should be
enabled.
--compiler=mercury
NOTE: this flag is REQUIRED for MercuryPIC binary
files to compile, as they contain different
delimiters and file endings than the rest of
the compilers listed above.