Bug 1490463 - part 1 - emit a srcdir_rel variable in every directory; r=mshal

Many Windows tools don't deal well with absolute paths for filenames,
oddly enough, so we want to pass relative paths into the source
directory where possible.  These relative paths also mean that they look
like "normal" unix paths to WSL and therefore don't require any special
handling.

python/mozbuild/mozbuild/backend/base.py

@@ -317,6 +317,7 @@ class BuildBackend(LoggingMixin):
             top_srcdir=obj.topsrcdir,
             topobjdir=obj.topobjdir,
             srcdir=srcdir,
+            srcdir_rel=mozpath.relpath(srcdir, mozpath.dirname(obj.output_path)),
             relativesrcdir=mozpath.relpath(srcdir, obj.topsrcdir) or '.',
             DEPTH=mozpath.relpath(obj.topobjdir, mozpath.dirname(obj.output_path)) or '.',
         )

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -1677,6 +1677,7 @@ class RecursiveMakeBackend(CommonBackend):
             pp.handleLine(b'topobjdir := @topobjdir@\n')
             pp.handleLine(b'topsrcdir := @top_srcdir@\n')
             pp.handleLine(b'srcdir := @srcdir@\n')
+            pp.handleLine(b'srcdir_rel := @srcdir_rel@\n')
             pp.handleLine(b'VPATH := @srcdir@\n')
             pp.handleLine(b'relativesrcdir := @relativesrcdir@\n')
             pp.handleLine(b'include $(DEPTH)/config/@autoconfmk@\n')

python/mozbuild/mozbuild/test/backend/test_recursivemake.py

@@ -225,6 +225,7 @@ class TestRecursiveMakeBackend(BackendTester):
             'topobjdir := %s' % env.topobjdir,
             'topsrcdir := %s' % env.topsrcdir,
             'srcdir := %s' % env.topsrcdir,
+            'srcdir_rel := %s' % mozpath.relpath(env.topsrcdir, env.topobjdir),
             'VPATH := %s' % env.topsrcdir,
             'relativesrcdir := .',
             'include $(DEPTH)/config/autoconf.mk',
@@ -242,7 +243,7 @@ class TestRecursiveMakeBackend(BackendTester):
         self.assertTrue(os.path.exists(p))
 
         lines = [l.strip() for l in open(p, 'rt').readlines()]
-        self.assertEqual(len(lines), 10)
+        self.assertEqual(len(lines), 11)
 
         self.assertTrue(lines[0].startswith('# THIS FILE WAS AUTOMATICALLY'))