tests, benchmark, tool makefile and less C lib

Add built-in PngSuite tests

Improve the benchmark tool

Add a Makefile for the tools (not intended for the library itself)

Avoid direct usage of memcpy, memset, strlen, strcmp: reimplement them
to avoid issues with C libraries lacking those on tiny platforms.

A few minor optimizations.

Makefile

@@ -0,0 +1,34 @@
+# This makefile only makes the unit test, benchmark and pngdetail and showpng
+# utilities. It does not make the PNG codec itself as shared or static library.
+# That is because:
+# LodePNG itself has only 1 source file (lodepng.cpp, can be renamed to
+# lodepng.c) and is intended to be included as source file in other projects and
+# their build system directly.
+
+
+CC ?= gcc
+CXX ?= g++
+
+override CFLAGS := -W -Wall -Wextra -ansi -pedantic -O3 -Wno-unused-function $(CFLAGS)
+override CXXFLAGS := -W -Wall -Wextra -ansi -pedantic -O3 $(CXXFLAGS)
+
+all: unittest benchmark pngdetail showpng
+
+%.o: %.cpp
+	@mkdir -p `dirname $@`
+	$(CXX) -I ./ $(CXXFLAGS) -c $< -o $@
+
+unittest: lodepng.o lodepng_util.o lodepng_unittest.o
+	$(CXX) $^ $(CXXFLAGS) -o $@
+
+benchmark: lodepng.o lodepng_benchmark.o
+	$(CXX) $^ $(CXXFLAGS) -lSDL -o $@
+
+pngdetail: lodepng.o lodepng_util.o pngdetail.o
+	$(CXX) $^ $(CXXFLAGS) -o $@
+
+showpng: lodepng.o examples/example_sdl.o
+	$(CXX) -I ./ $^ $(CXXFLAGS) -lSDL -o $@
+
+clean:
+	rm -f unittest benchmark pngdetail showpng lodepng_unittest.o lodepng_benchmark.o lodepng.o lodepng_util.o pngdetail.o examples/example_sdl.o