Make XTS build cleanly with gcc 14

Not perfectly clean, there's still thousands of compiler warnings, but no errors, and over 100k fewer compiler warnings than before this.

• Fix -Wimplicit-int errors in test framework
• Fix -Wimplicit-function-declaration errors in the test framework
• Fix -Wincompatible-pointer-types errors in the test framework
• Rcv_Ext_Evt: drop unused argument base
• Rcv_Ext_Evt: add missing argument to unpack2() call
• Xstlib.h: add guard to prevent multiple inclusion
• Viewport.c: stop using NULL for non-pointer values
• xim_cb.c: stop using NULL for non-pointer value
• Fix -Wimplicit-int errors in test cases
• Fix -Wimplicit-function-declaration errors in test cases
• Fix -Wint-conversion errors in generated code in test cases
• Xlib14 tests: stop using NULL for non-pointer values
• XtGetSelectionRequest.m: stop using NULL for non-pointer value
• vmath.h: Assume we want ANSI C function prototypes now
• Fix -Wincompatible-pointer-types errors in Xt test cases
• xtestlib.h: add guard to prevent multiple inclusion
• mc: always define tet_thistest as external in generated test code
• define tet_thistest as external in pre-written test code
• r5.h: remove extra global definition of drawable_id
• Mark keycode variables in keyboard tests as static
• define ebuf as extern in all but one file
• Remove unneeded declaration of s2 from libXtTest/waitfor.c
• Fix remaining multiply-defined linker errors in Xt test cases
• Fix most -Wdiscarded-qualifiers warnings in tet framework
• avsr5_proto.h: uncomment function prototypes
• Fix most -Wdiscarded-qualifiers warnings in xts5 framework
• Fix many -Wdiscarded-qualifiers warnings in test cases
• Fix -Wreturn-mismatch errors in test cases
• AvsForm2.h: avoid conflicts with XawForm.h
• Fix -Wmissing-prototypes warnings in tet framework
• Log.c: convert to actual varargs functions
• ShowRep.c: Add missing % to printf-style format string
• Fix -Wmissing-prototypes warnings in xts5 framework
• mc: use (void) instead of () for functions in generated code
• XChangeWindowAttributes: fix before & after test for y value
• Replace wbcopy() function with a macro that calls memmove()
• Replace xt_strdup() function with a macro that calls strdup()
• mc: Replace mcstrdup() function with a macro that calls strdup()
• XcmsStoreColor: check for success should use ==, not =
• Fix -Wreturn-type warnings in the test framework
• meson.build: Fix x11_trans.c compile on Solaris
• Rename delete() to xts_delete() [via #define]

Closes: #22 (closed), #23 (closed)