SunOS header workaround breaks compilation with gcc-15

[mschubert]

I'm getting the following compilation error on Archlinux with gcc-15:

read_boulder.c:106:15: error: conflicting types for ‘strtod’; have ‘double(void)’
  106 | extern double strtod();
      |               ^~~~~~
In file included from read_boulder.c:45:
/usr/include/stdlib.h:122:15: note: previous declaration of ‘strtod’ with type ‘double(const char *,  char **)’
  122 | extern double strtod (const char *__restrict __nptr,
      |               ^~~~~~
read_boulder.c: In function ‘parse_double’:
read_boulder.c:1001:12: error: too many arguments to function ‘strtod’; expected 0, have 2
 1001 |     *out = strtod(datum, &nptr);
      |            ^~~~~~ ~~~~~
read_boulder.c:106:15: note: declared here
  106 | extern double strtod();
      |               ^~~~~~
make: *** [Makefile:225: read_boulder.o] Error 1

This is the same as this Debian Bug, with the same solution: remove the old compatibility declaration.

--- a/src/read_boulder.c
+++ b/src/read_boulder.c
@@ -99,14 +99,6 @@
 static const char *pr_program_name = "TMP";
 
 /* 
- * Hack to support old SunOS headers.  (We do not try to declare _all_
- * undeclared functions; only those with non-int return types.)
- */
-#ifndef __cplusplus
-extern double strtod();
-#endif
-
-/* 
  * See read_boulder.h for description.
  */
 #define COMPARE(TAG) (!strncmp(s, TAG, tag_len) \