Don't override MacPorts environment variables.
Remove optimization flag so that the one from MacPorts is used.
--- config.mk.orig	2019-02-02 06:55:02.000000000 -0600
+++ config.mk	2020-06-15 19:12:12.000000000 -0500
@@ -23,9 +23,9 @@
 LIBS = -L$(X11LIB) -lX11 $(XINERAMALIBS) $(FREETYPELIBS)
 
 # flags
-CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200809L -DVERSION=\"$(VERSION)\" $(XINERAMAFLAGS)
-CFLAGS   = -std=c99 -pedantic -Wall -Os $(INCS) $(CPPFLAGS)
-LDFLAGS  = $(LIBS)
+CPPFLAGS += -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200809L -DVERSION=\"$(VERSION)\" $(XINERAMAFLAGS)
+CFLAGS   += -std=c99 -pedantic -Wall $(INCS) $(CPPFLAGS)
+LDFLAGS  += $(LIBS)
 
 # compiler and linker
 CC = cc