Makefile Techniques

Conditional Blocks (stackoverflow)

ifeq ($(TARGET), scmb)
  CC = gcc-riscv-eabi-none
  CC = gcc

In a style like you would expect from a scripting language, defining the variable inside the if doesn't prevent it from being accessible later. However, if you want to indent inside the if, make sure that it doesn't match the indentation used for commands inside a rule.

Automatically Build Header File Dependencies (stackoverflow)

CXX = clang++
CXX_FLAGS = -Wfatal-errors -Wall -Wextra -Wpedantic -Wconversion -Wshadow

# Final binary
BIN = mybin
# Put all auto generated stuff to this build dir.
BUILD_DIR = ./build

# List of all .cpp source files.
CPP = main.cpp $(wildcard dir1/*.cpp) $(wildcard dir2/*.cpp)

# All .o files go to build dir.
OBJ = $(CPP:%.cpp=$(BUILD_DIR)/%.o)
# Gcc/Clang will create these .d files containing dependencies.
DEP = $(OBJ:%.o=%.d)

# Default target named after the binary.
$(BIN) : $(BUILD_DIR)/$(BIN)

# Actual target of the binary - depends on all .o files.
$(BUILD_DIR)/$(BIN) : $(OBJ)
    # Create build directories - same structure as sources.
    mkdir -p $(@D)
    # Just link all the object files.
    $(CXX) $(CXX_FLAGS) $^ -o $@

# Include all .d files
-include $(DEP)

# Build target for every single object file.
# The potential dependency on header files is covered
# by calling `-include $(DEP)`.
$(BUILD_DIR)/%.o : %.cpp
    mkdir -p $(@D)
    # The -MMD flags additionaly creates a .d file with
    # the same name as the .o file.
    $(CXX) $(CXX_FLAGS) -MMD -c $< -o $@

.PHONY : clean
clean :
    # This should remove all generated files.
    -rm $(BUILD_DIR)/$(BIN) $(OBJ) $(DEP)

The important parts here are building the DEP list of .d files, and the -MMD flag to gcc in the rule for building .o files. This snippet also makes use of the -p flag to mkdir to not error on the directory already existing, but I prefer having a dedicated rule for mkdir, and having the other rules having the timeless dependency, like %.o: %.c | $(BUILD_DIR)