feat: restructure Makefile to delegate builds to script/wren.mk and output to bin/ and lib/ directories

The monolithic Makefile is replaced with a thin wrapper that invokes
script/wren.mk for each configuration. Build artifacts are now placed
in bin/ (executables) and lib/ (static/shared libraries) instead of
the project root. The .gitignore is updated accordingly, and the
getting-started documentation reflects the new output layout.

script/wren.mk

@@ -0,0 +1,108 @@
+# Makefile for building a single configuration of Wren. It allows the
+# following variables to be passed to it:
+#
+# MODE - The build mode, "debug" or "release".
+#				 If omitted, defaults to "release".
+# LANG - The language, "c" or "cpp".
+# 		   If omitted, defaults to "c".
+# ARCH - The processor architecture, "32", "64", or nothing, which indicates
+#				 the compiler's default.
+#        If omitted, defaults to the compiler's default.
+#
+# It builds a static library, shared library, and command-line interpreter for
+# the given configuration. Libraries are built to "lib", and the interpreter
+# is built to "bin".
+#
+# The output file is initially "wren". If in debug mode, "d" is appended to it.
+# If the language is "cpp", then "-cpp" is appended to that. If the
+# architecture is not the default then "-32" or "-64" is appended to that.
+# Then, for the libraries, the correct extension is added.
+
+# Files.
+HEADERS   := include/wren.h $(wildcard src/*.h)
+SOURCES   := $(wildcard src/*.c)
+BUILD_DIR := build
+
+CFLAGS := -Wall -Werror
+# TODO: Add -Wextra.
+
+# Mode configuration.
+ifeq ($(MODE),debug)
+	WREN := wrend
+	CFLAGS += -O0 -DDEBUG -g
+	BUILD_DIR := $(BUILD_DIR)/debug
+else
+	WREN += wren
+	CFLAGS += -Os
+	BUILD_DIR := $(BUILD_DIR)/release
+endif
+
+# Language configuration.
+ifeq ($(LANG),cpp)
+	WREN := $(WREN)-cpp
+	CFLAGS += -std=c++98
+	FILE_FLAG := -x c++
+	BUILD_DIR := $(BUILD_DIR)-cpp
+else
+	CFLAGS += -std=c99
+endif
+
+# Architecture configuration.
+ifeq ($(ARCH),32)
+	CFLAGS += -m32
+	WREN := $(WREN)-32
+	BUILD_DIR := $(BUILD_DIR)-32
+endif
+
+ifeq ($(ARCH),64)
+	CFLAGS += -m64
+	WREN := $(WREN)-64
+	BUILD_DIR := $(BUILD_DIR)-64
+endif
+
+# Don't add -fPIC on Windows since it generates a warning which gets promoted
+# to an error by -Werror.
+OS := $(lastword $(subst -, ,$(shell $(CC) -dumpmachine)))
+ifeq      ($(OS),mingw32)
+else ifeq ($(OS),cygwin)
+	# Do nothing.
+else
+	CFLAGS += -fPIC
+endif
+
+# Clang on Mac OS X has different flags and a different extension to build a
+# shared library.
+ifneq (,$(findstring darwin,$(OS)))
+	SHARED_EXT := dylib
+else
+	SHARED_LIB_FLAGS := -Wl,-soname,$@.so
+	SHARED_EXT := so
+endif
+
+# TODO: Simplify this if we mode main.c to a different directory.
+OBJECTS := $(addprefix $(BUILD_DIR)/, $(notdir $(SOURCES:.c=.o)))
+# Don't include main.c in the libraries.
+LIB_OBJECTS := $(subst $(BUILD_DIR)/main.o,,$(OBJECTS))
+
+# Targets ---------------------------------------------------------------------
+
+all: prep bin/$(WREN) lib/lib$(WREN).a lib/lib$(WREN).$(SHARED_EXT)
+
+prep:
+	@mkdir -p bin lib $(BUILD_DIR)
+
+# Command-line interpreter.
+bin/$(WREN): $(OBJECTS)
+	$(CC) $(CFLAGS) -Iinclude -o $@ $^ -lm
+
+# Static library.
+lib/lib$(WREN).a: $(LIB_OBJECTS)
+	$(AR) rcu $@ $^
+
+# Shared library.
+lib/lib$(WREN).$(SHARED_EXT): $(LIB_OBJECTS)
+	$(CC) $(CFLAGS) -shared $(SHARED_LIB_FLAGS) -o $@ $^
+
+# Object files.
+$(BUILD_DIR)/%.o: src/%.c $(HEADERS)
+	$(CC) -c $(CFLAGS) -Iinclude -o $@ $(FILE_FLAG) $<