In commit ac988cf we made sure to undefine the CONST/VOID macros left
defined by `tcl.h`, but this in turn makes it an issue to include
additional Tcl headers later on (see issue #4808).
One way out is to avoid a global `tcl.h` include. In the process we drop
support for Tcl-enabled MXE builds, which were likely broken anyway due
to the additional Tcl APIs used from `tclapi.cc`.
This adjusts the way the headers kernel/{yosys,rtlil,register,log}.h
include each other to avoid the need of including headers outside of
include guards as well as avoiding the inclusion of rtlil.h in the
middle of yosys.h with rtlil.h depending on the prefix of yosys.h, and
the suffix of yosys.h depending on rtlil.h.
To do this I moved some of the declaration in yosys.h into a new header
yosys_common.h. I'm not sure if that is strictly necessary.
Including any of these files still results in the declarations of all
these headers being included, so this shouldn't be a breaking change for
any passes or external plugins.
My main motivation for this is that ccls's (clang based language server)
include guard handling gets confused by the previous way the includes
were done. It often ends up treating the include guard as a generic
disabled preprocessor conditional, breaking navigation and highlighting
for the core RTLIL data structures.
Additionally I think avoiding cyclic includes in the middle of header
files that depend on includes being outside of include guards will also
be less confusing for developers reading the code, not only for tools
like ccls.
