build: store build.ninja outside build directory

This preserves configuration options after a make clean (and
also allows you to call ninja-build directly).

.gitignore

@@ -2,3 +2,4 @@
 .project
 .settings
 build
+build.ninja

Makefile

@@ -1,9 +1,9 @@
 out = build
 
-all: $(out)/build.ninja
+all: build.ninja
 	@ninja-build -f $<
 
-$(out)/build.ninja:
+build.ninja:
 	python3 configure.py
 
 clean:

configure.py

@@ -55,7 +55,7 @@ args = arg_parser.parse_args()
 globals().update(vars(args))
 
 outdir = 'build'
-buildfile = outdir + '/build.ninja'
+buildfile = 'build.ninja'
 os.makedirs(outdir, exist_ok = True)
 with open(buildfile, 'w') as f:
     f.write(textwrap.dedent('''\
@@ -94,5 +94,5 @@ with open(buildfile, 'w') as f:
         rule configure
           command = python3 configure.py $configure_args
           generator = 1
-        build $builddir/build.ninja: configure | configure.py
+        build build.ninja: configure | configure.py
         '''))