[Variadic] avoid musl-related test differences

src/plugins/variadic/tests/defined/oracle/simple.0.res.oracle

-[variadic] simple.c:9: Declaration of variadic function sum.
-[variadic] simple.c:24: Generic translation of call to variadic function.
+[variadic] simple.c:19: Declaration of variadic function sum.
+[variadic] simple.c:34: Generic translation of call to variadic function.
 [eva] Analyzing a complete application starting at main
 [eva] Computing initial state
 [eva] Initial state computed
@@ -12,7 +12,7 @@
 [eva:final-states] Values at end of function main:
   
 /* Generated by Frama-C */
-#include "stdarg.h"
+typedef void * const *va_list;
 /*@ requires n ≥ 0; */
 int sum(int n, void * const *__va_params)
 {

src/plugins/variadic/tests/defined/oracle/simple.1.res.oracle

-[variadic] simple.c:9: Declaration of variadic function sum.
-[variadic] simple.c:24: Generic translation of call to variadic function.
+[variadic] simple.c:19: Declaration of variadic function sum.
+[variadic] simple.c:34: Generic translation of call to variadic function.
 [eva] Analyzing a complete application starting at main
 [eva] Computing initial state
 [eva] Initial state computed
@@ -12,8 +12,7 @@
 [eva:final-states] Values at end of function main:
   
 /* Generated by Frama-C */
-typedef void * const *__gnuc_va_list;
-typedef __gnuc_va_list va_list;
+typedef void * const *va_list;
 /*@ requires n ≥ 0; */
 int sum(int n, void * const *__va_params)
 {

src/plugins/variadic/tests/defined/simple.c

 /* run.config
-STDOPT:
-STDOPT: #"-no-frama-c-stdlib -no-pp-annot"
+STDOPT: #"-c11"
+STDOPT: #"-no-frama-c-stdlib -no-pp-annot -c11"
 */
 
+/* The defines and typedefs below avoid issues with Musl: without them,
+   pretty-printing 'va_list' results in `typedef __gnuc_va_list va_list`, but
+   only on GNU libc-based systems (and not on Alpine Linux, which is based on
+   musl). */
+#define __GNUC_VA_LIST
+#define __va_list__
+typedef void *__builtin_va_list;
+typedef __builtin_va_list va_list;
 #include <stdarg.h>
 
+
+
 /*@ requires n>= 0; */
 int sum(int n, ...){
   int ret = 0;