Don't install cythonize dependencies unless cythonizing

Numpy is not building in my environment, but I should not need to have
it if all I want to build is the default libvmaf target.

Additionally:

Instead of using the mtime of the .venv directory as a marker for
whether meson and ninja were successfully installed, use their wrapper
scripts from .venv/bin.

The $(VENV) target uses set -e and escaped newlines to run all commands
in a single shell invocation. Unless these commands are modifying the
shell's environment, this is redundant and mimics the way make already
works when you put a single command on each line.

Author: sethk

Makefile

@@ -9,7 +9,8 @@ VIRTUAL_ENV_PATH := $(VENV)/bin
 PYTHON_INTERPRETER := python3.10
 VENV_PIP := $(VIRTUAL_ENV_PATH)/pip
 VENV_PYTHON := $(VIRTUAL_ENV_PATH)/python
-MESON_SETUP := $(VIRTUAL_ENV_PATH)/meson setup
+MESON := $(VIRTUAL_ENV_PATH)/meson
+MESON_SETUP := $(MESON) setup
 NINJA := $(VIRTUAL_ENV_PATH)/ninja
 
 # Build types and options
@@ -22,31 +23,31 @@ LIBVMAF_DIR := libvmaf
 BUILD_DIR := $(LIBVMAF_DIR)/build
 DEBUG_DIR := $(LIBVMAF_DIR)/debug
 
-.PHONY: default all debug build install cythonize clean distclean
+.PHONY: default all debug build install cythonize clean distclean cythonize-deps
 
 default: build
 
 all: build debug install test cythonize
 
-$(BUILD_DIR): $(VENV)
+$(BUILD_DIR): $(MESON) $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(MESON_SETUP) $(BUILD_DIR) $(LIBVMAF_DIR) $(BUILDTYPE_RELEASE) $(ENABLE_FLOAT)
 
-$(DEBUG_DIR): $(VENV)
+$(DEBUG_DIR): $(MESON) $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(MESON_SETUP) $(DEBUG_DIR) $(LIBVMAF_DIR) $(BUILDTYPE_DEBUG) $(ENABLE_FLOAT)
 
-cythonize: $(VENV)
+cythonize: cythonize-deps
 	pushd python && ../$(VENV_PYTHON) setup.py build_ext --build-lib . && popd || exit 1
 
-build: $(BUILD_DIR) $(VENV)
+build: $(BUILD_DIR) $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(NINJA) -vC $(BUILD_DIR)
 
-test: build $(VENV)
+test: build $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(NINJA) -vC $(BUILD_DIR) test
 
-debug: $(DEBUG_DIR) $(VENV)
+debug: $(DEBUG_DIR) $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(NINJA) -vC $(DEBUG_DIR)
 
-install: $(BUILD_DIR) $(VENV)
+install: $(BUILD_DIR) $(NINJA)
 	PATH="$(VENV)/bin:$$PATH" $(NINJA) -vC $(BUILD_DIR) install
 
 clean:
@@ -57,10 +58,17 @@ distclean: clean
 	rm -rf $(VENV)
 
 # Set up or rebuild virtual environment
-$(VENV):
+$(VENV_PIP):
 	@echo "Setting up the virtual environment..."
-	@set -e; \
-	$(PYTHON_INTERPRETER) -m venv $(VENV) || { echo "Failed to create virtual environment"; exit 1; }; \
-	$(VENV_PIP) install --upgrade pip || { echo "Failed to upgrade pip"; exit 1; }; \
-	$(VENV_PIP) install meson ninja cython numpy || { echo "Failed to install dependencies"; exit 1; }
-	@echo "Virtual environment setup complete."
+	$(PYTHON_INTERPRETER) -m venv $(VENV) || { echo "Failed to create virtual environment"; exit 1; }
+	$(VENV_PIP) install --upgrade pip || { echo "Failed to upgrade pip"; exit 1; }
+	@echo "Virtual environment setup complete."
+
+$(MESON): $(VENV_PIP)
+	$(VENV_PIP) install meson || { echo "Failed to install meson"; exit 1; }
+
+$(NINJA): $(VENV_PIP)
+	$(VENV_PIP) install ninja || { echo "Failed to install ninja"; exit 1; }
+
+cythonize-deps: $(VENV_PIP)
+	$(VENV_PIP) install setuptools cython numpy || { echo "Failed to install dependencies"; exit 1; }