Makefile made more generic

Uses pkg-config to source locations of libSDL.
As commented in Makefile, *may* cause issues with builds on Mac, but
there are known Mac-isms to make this work.
Also needs the math library for round() as referenced by video.o
This causes builds to start working on Ubuntu 24.04 with the default
SDL2 libraries installed using apt.
Arch is known to need pkg-config to use SDL libs too, but I'm not
sitting at my arch machine at the moment, so no idea if this is complete
for that platform.
A meta-build system *may* be a good idea?

Makefile

@@ -1,18 +1,24 @@
 # Variables
 CC = gcc
-CFLAGS = -Wall -Wextra -std=c11 -I/usr/local/include/SDL2
-LDFLAGS = -L/usr/local/lib -lSDL2
+# Probably this won't work on a mac using xcode.
+# See https://stackoverflow.com/questions/3071321/how-to-use-pkg-config-for-setting-include-paths-in-xcode
+# for possible workarounds?
+# != is a GNU make extension, so may also not work on *BSD
+SDL_CFLAGS!=pkg-config --cflags sdl2
+SDL_LFLAGS!=pkg-config --libs sdl2
+CFLAGS = -Wall -Wextra -std=c11 $(SDL_CFLAGS)
+LDFLAGS = $(SDL_LFLAGS) -lm
 SRC_DIR = src
 SOURCES = $(wildcard $(SRC_DIR)/*.c)
 OBJECTS = $(SOURCES:$(SRC_DIR)/%.c=%.o)
 TARGET = bsx
 
 # Default target
-all: $(TARGET)
+all: $(TARGET) Makefile
 
 # Linking
 $(TARGET): $(OBJECTS)
-	$(CC) $(LDFLAGS) -o $@ $^
+	$(CC) -o $@ $^ $(LDFLAGS)
 
 # Compiling
 %.o: $(SRC_DIR)/%.c