Makefile: let variables be overridable via environment. (#30)

Currently, variables can only be passed down and overridden by directly specifying them as parameters to the `make` call.

It's better to take values from the environment instead and only define variables if the environment doesn't already do so.

In order for this to work correctly, we also want to split up `LDFLAGS` into `LDFLAGS` and `LIBS`, so that overriding the `LDFLAGS` themselves do not touch the linked library list.

Makefile

@@ -1,15 +1,18 @@
+CC ?= gcc
+CFLAGS ?= -Wall -Wextra -Werror -pedantic -std=c99 -O2 -I /usr/X11R6/include
+LDFLAGS ?= -L /usr/X11R6/lib -s
+PREFIX ?= /usr
+BIN ?= $(PREFIX)/bin
+MAN ?= $(PREFIX)/share/man/man1
+INSTALL ?= install
+
 PROG = xsct
-CC = gcc
-CFLAGS = -Wall -Wextra -Werror -pedantic -std=c99 -O2 -I /usr/X11R6/include
-LDFLAGS = -L /usr/X11R6/lib -lX11 -lXrandr -lm -s
 SRCS = xsct.c
-PREFIX = /usr
-BIN = $(PREFIX)/bin
-MAN = $(PREFIX)/share/man/man1
-INSTALL = install
+
+LIBS = -lX11 -lXrandr -lm
 
 $(PROG): $(SRCS)
-	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS)
+	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS) $(LIBS)
 
 install: $(PROG) $(PROG).1
 	$(INSTALL) -d $(DESTDIR)$(BIN)

README.md

@@ -23,6 +23,66 @@ Minor modifications were made in order to get sct to:
 
 # Installation
 
+## Make-based
+
+On UNIX-based systems, a convenient method of building this software is using
+Make. Since the `Makefile` is simple and portable, both the BSD and
+[GNU make](https://www.gnu.org/software/make/) variants will have no problems
+parsing and executing it correctly.
+
+The simplest invocation is
+
+~~~sh
+make
+~~~
+
+which will use the default values for all flags as provided in the Makefile.
+
+Overriding any of the following variables is supported by exporting a variable
+with the same name and your desired content to the environment:
+
+  * `CC`
+  * `CFLAGS`
+  * `LDFLAGS`
+  * `PREFIX`
+  * `BIN` (the directory into which the resulting binary will be installed)
+  * `MAN` (the directory into which the man page will be installed)
+  * `INSTALL` (`install(1)` program used to create directories and copy files)
+
+Both example calls are equivalent and will build the software with the
+specified compiler and custom compiler flags:
+
+~~~sh
+make CC='clang' CFLAGS='-O2 -pipe -g3 -ggdb3' LDFLAGS='-L/very/special/directory -flto'
+~~~
+
+~~~sh
+export CC='clang'
+export CFLAGS='-O2 -pipe -g3 -ggdb3'
+export LDFLAGS='-L/very/special/directory -flto'
+make
+~~~
+
+Lastly, the software can be installed by calling
+
+~~~sh
+make install
+~~~
+
+Or, if you prefer a different installation location, override the `PREFIX`
+variable:
+
+~~~sh
+make install PREFIX="${HOME}/xsct-prefix"
+~~~
+
+~~~sh
+export PREFIX="${HOME}/xsct-prefix"
+make install
+~~~
+
+## Manual compilation
+
 Compile the code using the following command:
 ~~~sh
 gcc -Wall -Wextra -Werror -pedantic -std=c99 -O2 -I /usr/X11R6/include xsct.c -o xsct -L /usr/X11R6/lib -lX11 -lXrandr -lm -s