Do not let tmux pick the optimisation and debug level itself; that is what
CFLAGS is for and the ebuild already handles it (see the replace-flags call for
bug 438558).

This is app-misc/tmux/files/tmux-2.4-flags.patch from ::gentoo, rebased. The
old patch dropped "-O2" from an "AM_CFLAGS += -std=gnu99 -O2" line and a bare
"AM_CFLAGS += -g".  Upstream has since moved both: the optimisation level now
sits behind the IS_OPTIMIZED conditional, and -g became "-g3 -ggdb" under
IS_DEBUG.  The old hunk therefore no longer applies, but the reason for it did
not go away -- IS_OPTIMIZED defaults to yes (configure.ac, enable_optimizations=yes),
so an unpatched build still appends its own -O2.

--disable-optimizations is not a substitute: the else branch of that same
conditional appends -O0, which is strictly worse.

Removing the whole conditional leaves AM_CONDITIONAL(IS_OPTIMIZED) defined and
unused, which automake accepts (only the reverse, used-but-undefined, is an
error).
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,13 +21,7 @@
 # Set flags for gcc.
 if IS_GCC
 AM_CFLAGS += -std=gnu99
-if IS_OPTIMIZED
-AM_CFLAGS += -O2
-else
-AM_CFLAGS += -O0
-endif
 if IS_DEBUG
-AM_CFLAGS += -g3 -ggdb
 AM_CFLAGS += -Wno-long-long -Wall -W -Wformat=2 -Wno-use-after-free
 AM_CFLAGS += -Wmissing-prototypes -Wstrict-prototypes -Wmissing-declarations
 AM_CFLAGS += -Wwrite-strings -Wshadow -Wpointer-arith -Wsign-compare