Makefile - include files not found when building with multiple source files

Question

I have a make file:

# Makefile
CC = gcc
SRC = pixel.c
TARGET = pixel.exe
OBJECTS = bmp.o pixel.o
#CFLAGS_W = -Wall -Wl,-subsystem,windows
CFLAGS_W = -Wall
LFLAGS_W = -lSDL2
INCS_W = -I.\SDL2\include
LIBS_W = -L.\SDL2\lib\x64

all: $(TARGET)

$(TARGET): $(OBJECTS)
    $(CC) $(INCS_W) $(LIBS_W) $(CFLAGS_W) $(LFLAGS_W) -g -o $(TARGET) $(OBJECTS)

it will build bmp.c, but when it tries to build pixel.c I get the error:

pixel.c:6:10: fatal error: SDL.h: No such file or directory

however, pixel.c has been building fine all the time I was just building it on its own (before I added bmp.c), using the command:

all:
    $(CC) $(SRC) $(INCS_W) $(LIBS_W) $(CFLAGS_W) $(LFLAGS_W) -g -o $(TARGET)

I really struggle with makefiles because I just don't use them enough, but this doesn't really feel like a "make" problem.

I'm using mingw32-make and gcc in a windows sandbox.

EDIT:

I have added

.c.o:
    $(CC) $(INCS_W) $(LIBS_W) $(CFLAGS_W) $(LFLAGS_W) -g -c $< -o $@

to my makefile, which gets me a step further (I think), but now complains that all of the SDL functions are undefined

C:\Users\WDAGUtilityAccount\Desktop\Projects\Pixel>mingw32-make
gcc -I./SDL2/include -L./SDL2/lib/x64 -Wall -lSDL2 -g -c bmp.c -o bmp.o
gcc -I./SDL2/include -L./SDL2/lib/x64 -Wall -lSDL2 -g -c pixel.c -o pixel.o
gcc -I./SDL2/include -L./SDL2/lib/x64 -Wall -lSDL2 -g -o pixel.exe bmp.o pixel.o
pixel.o: In function `main':
C:\Users\WDAGUtilityAccount\Desktop\Projects\Pixel/pixel.c:119: undefined reference to `SDL_Init'
C:\Users\WDAGUtilityAccount\Desktop\Projects\Pixel/pixel.c:121: undefined reference to `SDL_GetError'
C:\Users\WDAGUtilityAccount\Desktop\Projects\Pixel/pixel.c:126: undefined reference to `SDL_CreateWindow'
C:\Users\WDAGUtilityAccount\Desktop\Projects\Pixel/pixel.c:131: undefined reference to `SDL_GetError'
...

I can get the whole project to compile if I just do this on its own:

SRC = bmp.c pixel.c
...
test:
    $(CC) $(SRC) $(INCS_W) $(LIBS_W) $(CFLAGS_W) $(LFLAGS_W) -g -o $(TARGET)

which is fine, it'll do, but it rebuilds everything every time, and I'd like to know why it isn't working anyway :)