Conditional directives allow different behavior based on variables or environment. Common: ifeq, ifneq, ifdef, ifndef.

ifeq (equal):

ifeq ($(OS), Linux)
    LDFLAGS = -lrt
else
    LDFLAGS = -framework CoreFoundation
endif

If $(OS) equals Linux, set Linux-specific flags. Otherwise, use macOS flags.

ifneq (not equal):

ifneq ($(PLATFORM), windows)
    CFLAGS = -fPIC
endif

If $(PLATFORM) is not windows, add position-independent code flag.

ifdef (defined):

ifdef DEBUG
    CFLAGS += -g -O0
else
    CFLAGS += -O3
endif

If DEBUG is defined (even if empty), use debug flags. Otherwise, optimize.

ifndef (not defined):

ifndef INSTALL_PREFIX
    INSTALL_PREFIX = /usr/local
endif

If INSTALL_PREFIX is not set, use a default.

Detecting the platform:

UNAME_S := $(shell uname -s)
ifeq ($(UNAME_S), Linux)
    LDLIBS = -lrt
endif
ifeq ($(UNAME_S), Darwin)
    LDLIBS = -framework CoreFoundation
endif

Detect the OS at make time and set appropriate flags.

Conditional compilation:

ifdef ENABLE_TESTS
    SRCS += test.c
    TEST_TARGET = run_tests
else
    TEST_TARGET =
endif
 
app: $(SRCS)
	gcc -o app $(SRCS)
 
test: $(TEST_TARGET)
	@echo "Tests skipped (use: make ENABLE_TESTS=1)"
 
$(TEST_TARGET): test.c
	./run_tests

Enable tests with make ENABLE_TESTS=1 test.

Nested conditionals:

ifdef RELEASE
    CFLAGS = -O3
    ifdef STRIP
        LDFLAGS += -s
    endif
else
    CFLAGS = -g -O0
endif

Conditionals can be nested. Use indentation for clarity.

Conditional includes:

ifdef CONFIG_FILE
    include $(CONFIG_FILE)
else
    include default_config.mk
endif

Include different configuration files based on conditions.

Example: Debug vs Release builds:

ifdef DEBUG
    CFLAGS = -g -O0 -DDEBUG
    TARGET = app_debug
else
    CFLAGS = -O3
    TARGET = app
endif
 
$(TARGET): main.o util.o
	gcc -o $@ main.o util.o
 
clean:
	rm -f *.o app app_debug

Build with make DEBUG=1 for debug, make for release.