1. We actually do want tmux.conf to go into ${prefix}/etc,
since the real /etc is not writeable on Android.
2. Skip both reallocarray and recallocarray checks

diff -u -r ../tmux-3.2/configure.ac ./configure.ac
--- ../tmux-3.2/configure.ac	2020-04-27 08:38:04.000000000 +0000
+++ ./configure.ac	2020-04-28 22:36:58.650391000 +0000
@@ -52,9 +52,6 @@
 PKG_PROG_PKG_CONFIG
 AC_USE_SYSTEM_EXTENSIONS
 
-# Default tmux.conf goes in /etc not ${prefix}/etc.
-test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
-
 # Is this --enable-debug?
 case "x$VERSION" in xnext*) enable_debug=yes;; esac
 AC_ARG_ENABLE(
@@ -157,22 +154,8 @@
 # Clang sanitizers wrap reallocarray even if it isn't available on the target
 # system. When compiled it always returns NULL and crashes the program. To
 # detect this we need a more complicated test.
-AC_MSG_CHECKING([for working reallocarray])
-AC_RUN_IFELSE([AC_LANG_PROGRAM(
-		[#include <stdlib.h>],
-		[return (reallocarray(NULL, 1, 1) == NULL);]
-	)],
-	AC_MSG_RESULT(yes),
-	[AC_LIBOBJ(reallocarray) AC_MSG_RESULT([no])]
-)
-AC_MSG_CHECKING([for working recallocarray])
-AC_RUN_IFELSE([AC_LANG_PROGRAM(
-		[#include <stdlib.h>],
-		[return (recallocarray(NULL, 1, 1, 1) == NULL);]
-	)],
-	AC_MSG_RESULT(yes),
-	[AC_LIBOBJ(recallocarray) AC_MSG_RESULT([no])]
-)
+AC_LIBOBJ(reallocarray)
+AC_LIBOBJ(recallocarray)
 
 # Look for clock_gettime. Must come before event_init.
 AC_SEARCH_LIBS(clock_gettime, rt)