fix: clang compilation warnings with -Wstrict-prototypes

[dpelle]

Change fixes this kind of compilation warnings with clang-17:

proto/if_python3.pro:13:20: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
   13 | int python3_version();
      |                    ^
      |                     void

[pheiduck]

Interesting how do you compile vim?
I have made a PR to get clang-17 on the CI but it runs fine on latest patch.:
#12745

[codecov]

Codecov Report

Merging #13166 (a410778) into master (ceee7a8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head a410778 differs from pull request most recent head 664fde4. Consider uploading reports for the commit 664fde4 to get more accurate results

@@            Coverage Diff             @@
##           master   #13166      +/-   ##
==========================================
- Coverage   82.13%   82.12%   -0.01%     
==========================================
  Files         160      160              
  Lines      195308   195308              
  Branches    43823    43823              
==========================================
- Hits       160412   160395      -17     
- Misses      22058    22075      +17     
  Partials    12838    12838              

Flag	Coverage Δ
huge-clang-Array	82.73% <100.00%> (-0.01%)	⬇️
linux	82.73% <100.00%> (-0.01%)	⬇️
mingw-x64-HUGE	76.71% <100.00%> (+<0.01%)	⬆️
mingw-x86-HUGE	77.22% <100.00%> (+<0.01%)	⬆️
windows	78.30% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/os_unix.c	66.97% <ø> (-0.05%)	⬇️
src/debugger.c	89.17% <100.00%> (ø)
src/if_python3.c	74.45% <100.00%> (ø)

... and 12 files with indirect coverage changes

[dpelle]

@pheiduck asked:

Interesting how do you compile vim?
I have made a PR to get clang-17 on the CI but it runs fine on latest patch.:
#12745

To reproduce the warning with clang-17, I had uncommented out this line to enable -Wall -Wmissing-prototypes:

$ git diff
diff --git a/src/Makefile b/src/Makefile
index 4d0a5ebad..a5c0d9dea 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -602,7 +602,7 @@ CClink = $(CC)
 #CFLAGS = -g -O2 -fno-strength-reduce -Wall -Wmissing-prototypes
 #CFLAGS = -g -Wall -Wmissing-prototypes
 #CFLAGS = -O6 -fno-strength-reduce -Wall -Wshadow -Wmissing-prototypes
-#CFLAGS = -g -DDEBUG -Wall -Wshadow -Wmissing-prototypes
+CFLAGS = -g -DDEBUG -Wall -Wshadow -Wmissing-prototypes
 #CFLAGS = -g -O2 '-DSTARTUPTIME="vimstartup"' -fno-strength-reduce -Wall -Wmissing-prototypes

 # Use this with GCC to check for mistakes, unused arguments, etc.

I now see that the warning is not specific to clang-17 actually, it also happens with clang-14, clang-15, gcc-9, gcc-11 for example, so I'll reword the commit message.

[ychin]

I think if certain warnings are important, such as -Wmissing-prototypes and -Wstrict-prototypes, we should turn them on in ci/config.mk.sed or ci/config.mk.gcc.sed or ci/config.mk.clang.sed. This way it will actually be checked in CI.

Otherwise it's just whack-a-mole every time. We should either decide if such warnings are important (in which case we check them in CI), or unimportant (in which case we don't bother fixing them).

[ychin]

Hmm, I tried adding this to CI and seems like GTK and Ruby headers don't really like them, so it would be a little annoying to just add them in unless we also add warning suppression when including those headers…