Adjust build for native Apple Silicon

Look for dependencies in $HOMEBREW_PREFIX and use `brew --prefix` if it
is not set.

contrib/config.make-Darwin

@@ -1,12 +1,16 @@
 # Example configuration for Darwin / macOS.
 
+ifeq ($(origin HOMEBREW_PREFIX), undefined)
+HOMEBREW_PREFIX := $(shell brew --prefix 2>/dev/null)
+endif
+
 # Work-around for Homebrew-based xmlto.
-export XML_CATALOG_FILES=/usr/local/etc/xml/catalog
+export XML_CATALOG_FILES=$(HOMEBREW_PREFIX)/etc/xml/catalog
 
 TIG_CFLAGS += -DHAVE_EXECINFO_H
 TIG_LDLIBS = -liconv
 
-NCURSES_DIR ?= $(wildcard /usr/local/opt/ncurses)
+NCURSES_DIR ?= $(wildcard $(HOMEBREW_PREFIX)/opt/ncurses)
 
 ifneq ($(NCURSES_DIR),)
 	TIG_NCURSES = -lncursesw
@@ -16,15 +20,15 @@ else
 	TIG_CPPFLAGS += -DHAVE_CURSES_H
 endif
 
-READLINE_DIR ?= $(wildcard /usr/local/opt/readline)
+READLINE_DIR ?= $(wildcard $(HOMEBREW_PREFIX)/opt/readline)
 
 ifneq ($(READLINE_DIR),)
 	TIG_LDLIBS += -lreadline
 	TIG_LDFLAGS += -L$(READLINE_DIR)/lib
 	TIG_CPPFLAGS += -I$(READLINE_DIR)/include -DHAVE_READLINE
 endif
 
-PCRE2_DIR ?= $(wildcard /usr/local/opt/pcre2)
+PCRE2_DIR ?= $(wildcard $(HOMEBREW_PREFIX)/opt/pcre2)
 
 ifneq ($(PCRE2_DIR),)
 	TIG_LDLIBS += -lpcre2-posix -lpcre2-8