provide declarations for strtod_l and family

these aliases were originally intended to be for ABI compatibility
only, but their presence caused regressions in broken gnulib-based
software whose configure scripts detect the existing of these
functions then use them without declarations, resulting in bogus
return values.

include/stdlib.h

@@ -149,6 +149,10 @@ int ptsname_r(int, char *, size_t);
 char *ecvt(double, int, int *, int *);
 char *fcvt(double, int, int *, int *);
 char *gcvt(double, int, char *);
+struct __locale_struct;
+float strtof_l(const char *__restrict, char **__restrict, struct __locale_struct *);
+double strtod_l(const char *__restrict, char **__restrict, struct __locale_struct *);
+long double strtold_l(const char *__restrict, char **__restrict, struct __locale_struct *);
 #endif
 
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)