Fix "llabs" warnings under MinGW

Some MinGW versions complain about "llabs" not being defined when
including "inttypes.h". Include "stdlib.h" before.
diff --git a/src/Charmonizer/Probe/Integers.c b/src/Charmonizer/Probe/Integers.c
index f894eed..8fec02e 100644
--- a/src/Charmonizer/Probe/Integers.c
+++ b/src/Charmonizer/Probe/Integers.c
@@ -587,6 +587,12 @@
     chaz_ConfWriter_start_module("IntegerFormatStrings");
 
     if (has_inttypes) {
+        if (chaz_CC_is_mingw()) {
+            /* Suppress warnings about undefined inline function `llabs`
+             * under MinGW.
+             */
+            chaz_ConfWriter_add_sys_include("stdlib.h");
+        }
         chaz_ConfWriter_add_sys_include("inttypes.h");
     }