Add a configure option --enable-force-getenv.

Allows the use of getenv() rather than secure_getenv() to read MALLOC_CONF.
This helps in situations where hosts are under full control, and setting
MALLOC_CONF is needed while also setuid.  Disabled by default.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -266,6 +266,12 @@
  */
 #undef JEMALLOC_READLINKAT
 
+/*
+ * If defined, use getenv() (instead of secure_getenv() or
+ * alternatives) to access MALLOC_CONF.
+ */
+#undef JEMALLOC_FORCE_GETENV
+
 /*
  * Darwin (OS X) uses zones to work around Mach-O symbol override shortcomings.
  */