Avoid implicit ints and implicit function declarations

[fweimer-rh]

These language features have been removed from C in 1999. Future compilers are likely to stop accepting these constructs by default.

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

[chrstphrchvz]

I concur that these issues should be addressed, as #79 and #80 are alternative approaches for some of them (e.g. adding #include <stdlib.h> rather than replacing exit(…) with return …).

Also note that if Perl/Tk ever intends to build with -std=c2x or later, then K&R-style definitions will have to be removed entirely.

[eli-schwartz]

It looks like a big part of the issue is actually just bundling a decrepit code copy of tk.