Fix Kerberos build on Alpine 3.23+ (GCC 15/C23)

Alpine 3.23 ships with GCC 15, which uses C23 as the default language
standard. In C23, empty parentheses `()` in function declarations mean
"no arguments" (like C++) instead of "unspecified arguments" (C17 and
earlier).

Several dependencies (MIT Kerberos 1.21.3, Cyrus SASL 2.1.28) use
old-style K&R declarations that conflict with proper prototypes,
causing build errors like:

  error: conflicting types for 'ss_delete_info_dir'; have 'void(void)'
  error: too many arguments to function 'MD5_memcpy'; expected 0

This fix adds -std=gnu17 globally in setup_musl_compiler() to force
C17 semantics for all dependency builds. This is fully backwards
compatible since GCC 8-14 already used gnu17 as the default.

Author: mensfeld

ext/build_linux_x86_64_musl.sh

@@ -135,12 +135,17 @@ Or install individually as needed."
 setup_musl_compiler() {
     # musl-specific compiler flags
     export CC="gcc"
-    export CFLAGS="-fPIC -O2 -static-libgcc"
-    export CXXFLAGS="-fPIC -O2 -static-libgcc" 
+    # GCC 15+ (Alpine 3.23+) uses C23 by default, where empty parentheses ()
+    # mean "no arguments" instead of "unspecified arguments". Several dependencies
+    # (MIT Kerberos, Cyrus SASL) use old-style K&R declarations incompatible with C23.
+    # Force C17 standard globally for all dependency builds.
+    # This is backwards compatible: GCC 8-14 used gnu17 as default anyway.
+    export CFLAGS="-fPIC -O2 -static-libgcc -std=gnu17"
+    export CXXFLAGS="-fPIC -O2 -static-libgcc"
     export CPPFLAGS=""
     export LDFLAGS="-static-libgcc"
 
-    log "Applied musl-specific compiler flags"
+    log "Applied musl-specific compiler flags (with C17 for GCC 15+ compatibility)"
 }
 
 # Build OpenSSL for musl