HOWTO build a statically linked tmux in one script (downloads and builds dependencies from source)

build-tmux.sh

#!/bin/bash
TARGETDIR=$1
if [ "$TARGETDIR" = "" ]; then
TARGETDIR=$(python -c 'import os; print os.path.realpath("local")')
fi
mkdir -p $TARGETDIR

libevent() {
  curl -LO https://github.com/libevent/libevent/releases/download/release-2.0.22-stable/libevent-2.0.22-stable.tar.gz
  tar -zxvf libevent-2.0.22-stable.tar.gz
  cd libevent-2.0.22-stable
  ./configure --prefix=$TARGETDIR && make && make install
  cd ..
}

ncurses() {
  curl -LO https://ftp.gnu.org/pub/gnu/ncurses/ncurses-6.0.tar.gz
  tar zxvf ncurses-6.0.tar.gz
  cd ncurses-6.0

  ./configure --prefix $TARGETDIR \
              --with-default-terminfo-dir=/usr/share/terminfo \
              --with-terminfo-dirs="/etc/terminfo:/lib/terminfo:/usr/share/terminfo" \
              --enable-pc-files \
              --with-pkg-config-libdir=$HOME/local/lib/pkgconfig \
  && make && make install
  cd ..
}

tmux() {
  curl -LO https://github.com/tmux/tmux/releases/download/2.3/tmux-2.3.tar.gz
  tar zxvf tmux-2.3.tar.gz
  cd tmux-2.3
  PKG_CONFIG_PATH=$TARGETDIR/lib/pkgconfig ./configure --enable-static --prefix=$TARGETDIR && make && make install
  cd ..
  cp $TARGETDIR/bin/tmux .
}

libevent
ncurses
tmux

Fixes that worked for me (Centos7) for future reference.

strsep

(The same from @markfaine above)
In file included from /usr/include/string.h:633:0,
                 from arguments.c:22:
compat.h:211:8: error: expected identifier or ‘(’ before ‘__extension__’
 char  *strsep(char **, const char *);
        ^
compat.h:211:8: error: expected identifier or ‘(’ before ‘)’ token
 char  *strsep(char **, const char *);
        ^

Caused by configure falsely assume strsep is not available, since attempt to use it failed to compile. But the reason it does not compile is not strsep, it is cannot find -lc.
From https://stackoverflow.com/a/16025272, simply yum install glibc-static

forkpty

(Same from https://github.com/tmux/tmux/issues/1729)
In file included from tmux.h:36,
                 from cmd-copy-mode.c:21:
compat.h:301:9: error: conflicting types for ‘forkpty’
 pid_t   forkpty(int *, char *, struct termios *, struct winsize *);
         ^~~~~~~
In file included from compat.h:109,
                 from tmux.h:36,
                 from cmd-copy-mode.c:21:
/usr/include/pty.h:39:12: note: previous declaration of ‘forkpty’ was here
 extern int forkpty (int *__amaster, char *__name,

Add --with-termlib to ncurses configure step.

i had a libevent compile error (something having to do with dereffing incomplete types) on SUSE 15.1. changing to libevent 2.1.12 solved it

An updated version for tmux 3.2 using musl libc: https://gist.github.com/zenofile/d2acef1a0423e5081e74162cd5a0ae2d.

This is excellent, and has saved me much trouble. Thank you.