Make sure common environment variables (CC, CFLAGS, LDFLAGS, …) are not
overwritten. Also allow configuring the X11 library and include
directory separately.

diff -upr drawterm-0_hg20210414.orig/Make.unix drawterm-0_hg20210414/Make.unix
--- drawterm-0_hg20210414.orig/Make.unix	2021-04-20 18:51:54.514645075 +0200
+++ drawterm-0_hg20210414/Make.unix	2021-04-20 18:56:30.207396107 +0200
@@ -5,13 +5,16 @@ AR=ar
 AS=as
 RANLIB=ranlib
 X11=/usr/X11R6
-CC=gcc
-CFLAGS=-Wall -Wno-missing-braces -ggdb -I$(ROOT) -I$(ROOT)/include -I$(ROOT)/kern -c -I$(X11)/include -D_THREAD_SAFE $(PTHREAD) -O2
+X11INC=$(X11)/include
+X11LIB=$(X11)/lib
+CC?=gcc
+CFLAGS?=-O2
+CFLAGS+=-Wall -Wno-missing-braces -ggdb -I$(ROOT) -I$(ROOT)/include -I$(ROOT)/kern -c -I$(X11INC) -D_THREAD_SAFE $(PTHREAD)
 O=o
 OS=posix
 GUI=x11
-LDADD=-L$(X11)/lib64 -L$(X11)/lib -lX11 -ggdb -lm
-LDFLAGS=$(PTHREAD)
+LDADD=-L$(X11LIB) -lX11 -ggdb -lm
+LDFLAGS+=$(PTHREAD)
 TARG=drawterm
 # AUDIO=none
 AUDIO=unix