moved to POSIX-compatible make

Makefile

@@ -1,6 +1,43 @@
-CC = gcc
-CFLAGS = -Wall -Wextra -O2 -std=c99 -march=native $(shell pkg-config --cflags libcurl json-c readline)
-LDLIBS = $(shell pkg-config --libs libcurl json-c readline)
+# Makefile for Solo-C.
+#
+# Written for the POSIX make utility (IEEE Std 1003.1, XCU make) and a POSIX
+# shell (/bin/sh), so it works with any conforming make and no GNU-only
+# features. Deliberately avoided:
+#
+#   $(shell ...)     not POSIX - pkg-config is run by the recipe shell
+#   ?=               not POSIX - defaults are plain "=" and are overridden on
+#                           the command line, e.g. make CC=clang CFLAGS=-O1
+#   .PHONY           not POSIX - see the FORCE target at the bottom
+#   target-specific variables, ifeq, wildcard, -include, ... all non-POSIX
+#
+# Rule syntax used here: recipe lines start with a TAB, macros are written
+# $(NAME) (POSIX also accepts ${NAME}), and the only automatic macro used is
+# $@, the name of the target. Prerequisite names are written out: $< is POSIX
+# for inference rules only, and "$< is the first prerequisite" in a plain rule
+# is a GNU extension that even BusyBox make lacks.
+
+SHELL = /bin/sh
+
+# Standard macros. POSIX defaults CC to c99; Solo-C uses the system compiler
+# driver "cc" instead, which is the compiler a platform actually ships
+# (gcc, clang, musl-gcc, ...). Every macro here can be overridden from the
+# command line: make CC=clang, make CFLAGS=-O1, make LDLIBS=-lcurl.
+CC = cc
+CFLAGS = -Wall -Wextra -O2 -std=c99 -march=native
+LDFLAGS =
+LDLIBS =
+LINT = lint
+LINTFLAGS =
+LINTLIBS = -lc
+
+# Link-time discovery of the shared dependencies. POSIX make cannot run
+# commands while reading a makefile, so the flags are collected by the shell
+# that runs the recipe instead of by $(shell ...) while parsing.
+PKG_CONFIG = pkg-config
+PKGS = libcurl json-c readline
+# $$(...) survives expansion of the macro and reaches the shell as $(...).
+PKG_CFLAGS = $$($(PKG_CONFIG) --cflags $(PKGS))
+PKG_LIBS = $$($(PKG_CONFIG) --libs $(PKGS))
 
 # Fully static binary.
 #
@@ -21,41 +58,52 @@ LDLIBS = $(shell pkg-config --libs libcurl json-c readline)
 #
 # Archive order matters: libcurl.a pulls in ares_* symbols, so libcares.a has to
 # come after it on the link line.
-CURL_STATIC ?= static-deps/libcurl.a
-CARES_STATIC ?= static-deps/libcares.a
+CURL_STATIC = static-deps/libcurl.a
+CARES_STATIC = static-deps/libcares.a
 STATIC_LIBS = $(CURL_STATIC) $(CARES_STATIC) -lssl -lcrypto -lz -lidn2 -lunistring \
 	-lreadline -lncursesw -ljson-c -lm -ldl -pthread
 
 solo: main.c
-	$(CC) $(CFLAGS) -o solo main.c $(LDLIBS)
-	strip solo
+	$(CC) $(CFLAGS) $(PKG_CFLAGS) $(LDFLAGS) -o $@ main.c $(PKG_LIBS) $(LDLIBS)
+	strip $@
 
+# Writes solo-static, not a file called "static", so $@ is not used here.
 static: main.c
 	@for f in $(CURL_STATIC) $(CARES_STATIC); do \
 		test -f $$f || { echo "missing $$f - run: make static-deps"; exit 1; }; \
 	done
-	$(CC) $(CFLAGS) -fno-lto -static -o solo-static main.c $(STATIC_LIBS)
+	$(CC) $(CFLAGS) $(LDFLAGS) -fno-lto -static -o solo-static main.c $(STATIC_LIBS) $(LDLIBS)
 	strip solo-static
 
-static-deps:
+static-deps: FORCE
 	sh ./mkstaticdeps.sh
 
 # Rebuild both archives from scratch, ignoring what is already there.
-static-deps-force:
+static-deps-force: FORCE
 	FORCE=1 sh ./mkstaticdeps.sh
 
-PREFIX ?= /usr/local
+# Install paths. DESTDIR and PREFIX may be set on the command line:
+# make install DESTDIR=/tmp/pkg PREFIX=/usr
+PREFIX = /usr/local
 BINDIR = $(PREFIX)/bin
-INSTALL ?= install
+INSTALL = install
 
-install: solo
+install: solo FORCE
 	$(INSTALL) -d $(DESTDIR)$(BINDIR)
 	$(INSTALL) -m 755 solo $(DESTDIR)$(BINDIR)/solo
 
-uninstall:
+uninstall: FORCE
 	rm -f $(DESTDIR)$(BINDIR)/solo $(DESTDIR)$(BINDIR)/solo-static
 
-clean:
+clean: FORCE
 	rm -f solo solo-static
 
-.PHONY: clean static static-deps static-deps-force install uninstall
+# POSIX lint target: runs the lint tool named by the LINT macro.
+lint: main.c FORCE
+	$(LINT) $(LINTFLAGS) $(LINTLIBS) main.c
+
+# POSIX make has no .PHONY special target. Any target that depends on FORCE is
+# always out of date, because FORCE never creates a file, so its recipe always
+# runs. Keep this target last: the first target in the file is the default goal.
+FORCE:
+	@: