Parse number locale-independently

Previously we were using glibc's strtod function to parse floating point
numbers. The problem is, strtod is locale dependent. Meaning 7,7 might
be parsed as two numbers (7 and 7) in one locale, and parsed as one
number (7 point 7) in another locale. This is undesirable.

We need to set the locale to a value we know to make number parsing
consistently. We could use setlocale(), but that is not thread-safe. We
can also use uselocale(), which is thread-safe, but doesn't cover strtod
(Yeah, some of the locale-aware functions only acknowledge the global
locale, not the thread local one).

So in frustration, I just wrote a simple floating point number parser
myself. This parser obviously doesn't cover all cases strtod covers, but
is good enough for our needs.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/config.c

@@ -47,8 +47,8 @@ parse_long(const char *s, long *dest) {
  */
 const char *
 parse_matrix_readnum(const char *src, double *dest) {
-  char *pc = NULL;
-  double val = strtod(src, &pc);
+  const char *pc = NULL;
+  double val = strtod_simple(src, &pc);
   if (!pc || pc == src) {
     log_error("No number found: %s", src);
     return src;