patch 8.0.1832: cannot use :unlet for an environment variable

Problem:    Cannot use :unlet for an environment variable.
Solution:   Make it work.  Use unsetenv() if available. (Ken Takata,
            closes #2855)
diff --git a/src/config.h.in b/src/config.h.in
index 7d61220..00117cf 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -211,6 +211,7 @@
 #undef HAVE_TOWLOWER
 #undef HAVE_TOWUPPER
 #undef HAVE_ISWUPPER
+#undef HAVE_UNSETENV
 #undef HAVE_USLEEP
 #undef HAVE_UTIME
 #undef HAVE_BIND_TEXTDOMAIN_CODESET