[gdb/testsuite] Handle for loop initial decl with gcc 4.8.5

When running test-case gdb.threads/schedlock-thread-exit.exp on a system with
system compiler gcc 4.8.5, I run into:
...
src/gdb/testsuite/gdb.threads/schedlock-thread-exit.c:33:3: error: \
  'for' loop initial declarations are only allowed in C99 mode
...

Fix this by:
- using -std=c99, or
- using -std=gnu99, in case that's required, or
- in the case of the jit test-cases, rewriting the for loops.

Tested on x86_64-linux, both with gcc 4.8.5 and gcc 7.5.0.

gdb/testsuite/gdb.base/inferior-args.exp

@@ -23,7 +23,8 @@ if [target_info exists noargs] then {
 
 standard_testfile .c
 
-if {[build_executable "failed to prepare" $testfile $srcfile debug] == -1} {
+if {[build_executable "failed to prepare" $testfile $srcfile \
+	 {debug additional_flags=-std=c99}] == -1} {
     return
 }
 

gdb/testsuite/gdb.base/jit-elf-util.h

@@ -54,8 +54,8 @@ load_symbol (void *addr, const char *sym_name)
   ElfW (Addr) sym_new_addr = 0;
 
   /* Find `func_name` in symbol_table and return its address.  */
-
-  for (int i = 0; i < ehdr->e_shnum; ++i)
+  int i;
+  for (i = 0; i < ehdr->e_shnum; ++i)
     {
       if (shdr[i].sh_type == SHT_SYMTAB)
 	{
@@ -65,7 +65,8 @@ load_symbol (void *addr, const char *sym_name)
 	  char *const strtab
 	      = (char *) (addr + shdr[shdr[i].sh_link].sh_offset);
 
-	  for (ElfW (Sym) *p = symtab; p < symtab_end; ++p)
+	  ElfW (Sym) *p;
+	  for (p = symtab; p < symtab_end; ++p)
 	    {
 	      const char *s = strtab + p->st_name;
 	      if (strcmp (s, sym_name) == 0)

gdb/testsuite/gdb.base/run-attach-while-running.exp

@@ -106,7 +106,8 @@ proc_with_prefix test { non-stop threaded run-or-attach } {
 }
 
 foreach_with_prefix threaded {0 1} {
-    set options [list debug additional_flags=-DWITH_THREADS=$threaded]
+    set options [list debug additional_flags=-DWITH_THREADS=$threaded \
+		    additional_flags=-std=gnu99]
     if { $threaded } {
 	set binfile $binfile_threads
 	lappend options pthreads

gdb/testsuite/gdb.mi/interrupt-thread-group.exp

@@ -24,7 +24,7 @@ set MIFLAGS "-i=mi"
 standard_testfile .c
 
 if {[gdb_compile_pthreads "$srcdir/$subdir/$srcfile" $binfile \
-	executable debug] != "" } {
+	 executable {debug additional_flags=-std=gnu99}] != "" } {
     return -1
 }
 

gdb/testsuite/gdb.python/py-format-string.exp

@@ -33,7 +33,13 @@ if { [skip_python_tests] } { continue }
 proc build_inferior {exefile lang} {
   global srcdir subdir srcfile testfile hex
 
-  if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${exefile}" executable "debug $lang"] != "" } {
+  set flags [list debug $lang]
+  if { $lang == "c" } {
+    lappend flags additional_flags=-std=c99
+  }
+
+  if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${exefile}" \
+	    executable $flags] != "" } {
       untested "failed to compile in $lang mode"
       return -1
   }

gdb/testsuite/gdb.threads/multiple-successive-infcall.exp

@@ -23,7 +23,7 @@ if [get_compiler_info] {
 }
 
 if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
-  executable {debug}] != "" } {
+  executable {debug additional_flags=-std=gnu99}] != "" } {
   return -1
 }
 

gdb/testsuite/gdb.threads/schedlock-thread-exit.exp

@@ -22,7 +22,8 @@
 
 standard_testfile
 
-if { [build_executable "failed to prepare" ${testfile} ${srcfile} {debug pthreads}] } {
+if { [build_executable "failed to prepare" ${testfile} ${srcfile} \
+	  {debug pthreads additional_flags=-std=c99}] } {
     return
 }