build: make makefile work on windows

Using powershell as the default windows shell as using cmd alters $PATH
in a way that makes building neovim fail (powershell prioritizes visual
studio tools which is arguably more correct).

This was tested with gnu make for windows, which can be installed with
e.g. scoop. It does not work with nmake and it is extremely unlikely we
want to add nmake support as the makefile is merely supposed to be
syntactic sugar for the most common case. For similar reasons, the only
supported generator is ninja.

Makefile

@@ -1,3 +1,25 @@
+ifeq ($(OS),Windows_NT)
+  SHELL := powershell.exe
+  .SHELLFLAGS := -NoProfile -NoLogo
+  MKDIR := @$$null = new-item -itemtype directory -force
+  TOUCH := @$$null = new-item -force
+  RM := remove-item -force
+  CMAKE := cmake
+  CMAKE_GENERATOR := Ninja
+  define rmdir
+    if (Test-Path $1) { remove-item -recurse $1 }
+  endef
+else
+  MKDIR := mkdir -p
+  TOUCH := touch
+  RM := rm -rf
+  CMAKE := $(shell (command -v cmake3 || echo cmake))
+  CMAKE_GENERATOR ?= $(shell (command -v ninja > /dev/null 2>&1 && echo "Ninja") || echo "Unix Makefiles")
+  define rmdir
+    rm -rf $1
+  endef
+endif
+
 MAKEFILE_PATH := $(abspath $(lastword $(MAKEFILE_LIST)))
 MAKEFILE_DIR  := $(dir $(MAKEFILE_PATH))
 
@@ -9,7 +31,6 @@ filter-true = $(strip $(filter-out 1 on ON true TRUE,$1))
 
 all: nvim
 
-CMAKE ?= $(shell (command -v cmake3 || echo cmake))
 CMAKE_FLAGS := -DCMAKE_BUILD_TYPE=$(CMAKE_BUILD_TYPE)
 # Extra CMake flags which extend the default set
 CMAKE_EXTRA_FLAGS ?=
@@ -37,21 +58,11 @@ else
 checkprefix: ;
 endif
 
-CMAKE_GENERATOR ?= $(shell (command -v ninja > /dev/null 2>&1 && echo "Ninja") || \
-    echo "Unix Makefiles")
-DEPS_BUILD_DIR ?= .deps
+DEPS_BUILD_DIR ?= ".deps"
 ifneq (1,$(words [$(DEPS_BUILD_DIR)]))
   $(error DEPS_BUILD_DIR must not contain whitespace)
 endif
 
-ifeq (,$(BUILD_TOOL))
-  ifeq (Ninja,$(CMAKE_GENERATOR))
-    BUILD_TOOL = ninja
-  else
-    BUILD_TOOL = $(MAKE)
-  endif
-endif
-
 DEPS_CMAKE_FLAGS ?=
 USE_BUNDLED ?=
 
@@ -61,42 +72,41 @@ endif
 
 ifneq (,$(findstring functionaltest-lua,$(MAKECMDGOALS)))
   BUNDLED_LUA_CMAKE_FLAG := -DUSE_BUNDLED_LUA=ON
-  $(shell [ -x $(DEPS_BUILD_DIR)/usr/bin/lua ] || rm build/.ran-*)
+  $(shell [ -x $(DEPS_BUILD_DIR)/usr/bin/lua ] || $(RM) build/.ran-*)
 endif
 
 # For use where we want to make sure only a single job is run.  This does issue 
 # a warning, but we need to keep SCRIPTS argument.
 SINGLE_MAKE = export MAKEFLAGS= ; $(MAKE)
 
 nvim: build/.ran-cmake deps
-	$(BUILD_TOOL) -C build
+	$(CMAKE) --build build
 
 libnvim: build/.ran-cmake deps
-	$(BUILD_TOOL) -C build libnvim
+	$(CMAKE) --build build --target libnvim
 
 cmake:
-	touch CMakeLists.txt
+	$(TOUCH) CMakeLists.txt
 	$(MAKE) build/.ran-cmake
 
 build/.ran-cmake: | deps
-	$(CMAKE) -B build -G '$(CMAKE_GENERATOR)' $(CMAKE_FLAGS) $(CMAKE_EXTRA_FLAGS) $(MAKEFILE_DIR)
-	touch $@
+	$(CMAKE) -B build -G $(CMAKE_GENERATOR) $(CMAKE_FLAGS) $(CMAKE_EXTRA_FLAGS) $(MAKEFILE_DIR)
+	$(TOUCH) $@
 
 deps: | build/.ran-deps-cmake
 ifeq ($(call filter-true,$(USE_BUNDLED)),)
-	$(BUILD_TOOL) -C $(DEPS_BUILD_DIR)
+	$(CMAKE) --build $(DEPS_BUILD_DIR)
 endif
 
 ifeq ($(call filter-true,$(USE_BUNDLED)),)
 $(DEPS_BUILD_DIR):
-	mkdir -p "$@"
+	$(MKDIR) $@
 build/.ran-deps-cmake:: $(DEPS_BUILD_DIR)
-	$(CMAKE) -S $(MAKEFILE_DIR)/cmake.deps -B $(DEPS_BUILD_DIR) -G '$(CMAKE_GENERATOR)' \
-		$(BUNDLED_CMAKE_FLAG) $(BUNDLED_LUA_CMAKE_FLAG) $(DEPS_CMAKE_FLAGS)
+	$(CMAKE) -S $(MAKEFILE_DIR)/cmake.deps -B $(DEPS_BUILD_DIR) -G $(CMAKE_GENERATOR) $(BUNDLED_CMAKE_FLAG) $(BUNDLED_LUA_CMAKE_FLAG) $(DEPS_CMAKE_FLAGS)
 endif
 build/.ran-deps-cmake::
-	mkdir -p build
-	touch $@
+	$(MKDIR) build
+	$(TOUCH) "$@"
 
 # TODO: cmake 3.2+ add_custom_target() has a USES_TERMINAL flag.
 oldtest: | nvim
@@ -113,7 +123,7 @@ test/old/testdir/%.vim: phony_force nvim
 	$(SINGLE_MAKE) -C test/old/testdir NVIM_PRG=$(NVIM_PRG) SCRIPTS= $(MAKEOVERRIDES) $(patsubst test/old/testdir/%.vim,%,$@)
 
 functionaltest-lua: | nvim
-	$(BUILD_TOOL) -C build functionaltest
+	$(CMAKE) --build build --target functionaltest
 
 FORMAT=formatc formatlua format
 LINT=lintlua lintsh lintc clang-analyzer lintcommit lintdoc lint
@@ -135,16 +145,19 @@ iwyu: build/.ran-cmake
 	$(CMAKE) --build build
 
 clean:
-	test -d build && $(BUILD_TOOL) -C build clean || true
+ifneq ($(wildcard build),)
+	$(CMAKE) --build build --target clean
+endif
 	$(MAKE) -C test/old/testdir clean
 	$(MAKE) -C runtime/indent clean
 
 distclean:
-	rm -rf $(DEPS_BUILD_DIR) build
+	$(call rmdir, $(DEPS_BUILD_DIR))
+	$(call rmdir, build)
 	$(MAKE) clean
 
 install: checkprefix nvim
-	$(BUILD_TOOL) -C build install
+	$(CMAKE) --install build
 
 appimage:
 	bash scripts/genappimage.sh