https://sourceforge.net/p/wmweatherplus/bugs/21/
https://bugs.gentoo.org/870958

Port the configure script to C99 by avoiding calls to the undeclared
exit function.  Future compilers are likely to reject implicit
function declarations by default; this C features was removed from the
language in 1999.

--- a/m4/snprintf.m4
+++ b/m4/snprintf.m4
@@ -24,7 +24,7 @@ int snprintf(char *str, size_t size, const char *format, ...);
 #endif
 ]],
 [[char foo[]="ABC"; snprintf(foo, 2, "%d", 12);
-exit((foo[0]=='1' && foo[1]=='\0' && foo[2]=='C')?0:1);]])],
+return (foo[0]=='1' && foo[1]=='\0' && foo[2]=='C')?0:1;]])],
 [x_cv_func_snprintf_size=yes],
 [x_cv_func_snprintf_size=no],
 [x_cv_func_snprintf_size=no])])
@@ -52,7 +52,7 @@ AC_CACHE_CHECK([if snprintf return value is sane], x_cv_func_snprintf_retval,
 int snprintf(char *str, size_t size, const char *format, ...);
 #endif
 ]],
-[[char foo[10]; exit((snprintf(foo, 1, "%d", 9876)==4)?0:1);]])],
+[[char foo[10]; return (snprintf(foo, 1, "%d", 9876)==4)?0:1;]])],
 [x_cv_func_snprintf_retval=yes],
 [x_cv_func_snprintf_retval=no],
 [x_cv_func_snprintf_retval=no])])
@@ -79,7 +79,7 @@ AC_CACHE_CHECK([if snprintf(NULL, 0, ...) works], x_cv_func_snprintf_null_ok,
 int snprintf(char *str, size_t size, const char *format, ...);
 #endif
 ]],
-[int r=snprintf(NULL, 0, "%d", 100); exit((r==3 || r==-1)?0:1);])],
+[int r=snprintf(NULL, 0, "%d", 100); return (r==3 || r==-1)?0:1;])],
 [x_cv_func_snprintf_null_ok=yes],
 [x_cv_func_snprintf_null_ok=no],
 [x_cv_func_snprintf_null_ok=no])])
--- a/m4/vsnprintf.m4
+++ b/m4/vsnprintf.m4
@@ -37,7 +37,7 @@ int doit(char *str, size_t size, const char *format, ...){
 }
 ]],
 [[char foo[]="ABC"; doit(foo, 2, "%d", 12);
-exit((foo[0]=='1' && foo[1]=='\0' && foo[2]=='C')?0:1);]])],
+return (foo[0]=='1' && foo[1]=='\0' && foo[2]=='C')?0:1;]])],
 [x_cv_func_vsnprintf_size=yes],
 [x_cv_func_vsnprintf_size=no],
 [x_cv_func_vsnprintf_size=no])])
@@ -74,7 +74,7 @@ int doit(char *str, size_t size, const char *format, ...){
     return r;
 }
 ]],
-[[char foo[10]; exit((doit(foo, 1, "%d", 9876)==4)?0:1);]])],
+[[char foo[10]; return (doit(foo, 1, "%d", 9876)==4)?0:1;]])],
 [x_cv_func_vsnprintf_retval=yes],
 [x_cv_func_vsnprintf_retval=no],
 [x_cv_func_vsnprintf_retval=no])])
@@ -110,7 +110,7 @@ int doit(char *str, size_t size, const char *format, ...){
     return r;
 }
 ]],
-[int r=doit(NULL, 0, "%d", 100); exit((r==3 || r==-1)?0:1);])],
+[int r=doit(NULL, 0, "%d", 100); return (r==3 || r==-1)?0:1;])],
 [x_cv_func_vsnprintf_null_ok=yes],
 [x_cv_func_vsnprintf_null_ok=no],
 [x_cv_func_vsnprintf_null_ok=no])])