Preserve existing AM_CPPFLAGS and AM_LDFLAGS values.

Makefile.am

@@ -13,7 +13,7 @@ dist-hook:
 	grep "^#found_debug=" configure
 
 # Preprocessor flags.
-AM_CPPFLAGS = @XOPEN_DEFINES@ -DTMUX_CONF="\"$(sysconfdir)/tmux.conf\""
+AM_CPPFLAGS += @XOPEN_DEFINES@ -DTMUX_CONF="\"$(sysconfdir)/tmux.conf\""
 
 # glibc as usual does things ass-backwards and hides useful things by default,
 # so everyone has to add this.

configure.ac

@@ -52,7 +52,7 @@ AC_ARG_ENABLE(
 )
 if test "x$enable_static" = xyes; then
 	test "x$PKG_CONFIG" != x && PKG_CONFIG="$PKG_CONFIG --static"
-	AM_LDFLAGS="-static"
+	AM_LDFLAGS="$AM_LDFLAGS -static"
 fi
 
 # Is this gcc?