Add support for prepend/append in target type/pattern-specific vars

Semantically, these are similar to variable overrides and are essentially
treated as "templates" that are applied on lookup to the "stem" value that is
specific to the target type/name. For example:

x = [string] a
file{f*}: x =+ b

sub/:
{
  file{*}: x += c

  print $(file{foo}:x)  # abc
  print $(file{bar}:x)  # ac
}

3 files changed, 69 insertions, 0 deletions

diff --git a/tests/variable/type-pattern-append/buildfile b/tests/variable/type-pattern-append/buildfile
new file mode 100644
index 0000000..a91b340
--- /dev/null
+++ b/tests/variable/type-pattern-append/buildfile
@@ -0,0 +1,59 @@
+# Typed append/prepend.
+#
+#dir{a*}: x += [bool] true
+#dir{p*}: x =+ [bool] true
+
+[string] typed = [null]
+dir{a*}: typed += abc    # ok
+dir{p*}: typed =+ abc    # ok
+
+# Prepend/append before/after assignment.
+#
+[string] x1 = [null]
+dir{*}: x1 += A
+dir{*}: x1 = b
+dir{*}: x1 += c
+dir{*}: x1 =+ a
+print $(dir{./}:x1)
+
+# Without stem, mixed prepend/append.
+#
+dir{*}: x2 += b
+dir{*}: x2 += c
+#dir{*}: x2 =+ a     # error
+print $(dir{./}:x2)
+
+dir{*}: x3 =+ b
+dir{*}: x3 =+ a
+#dir{*}: x3 += c     # error
+print $(dir{./}:x3)
+
+# With stem, typing.
+#
+x4 = a
+dir{*}: x4 += b
+dir{*}: x4 += c
+print $(dir{./}:x4)
+
+[string] x5 = b
+dir{*}: x5 =+ a
+x = $(dir{./}:x5)
+print $(dir{./}:x5)
+
+x6 = [string] a
+sub/:
+{
+  dir{*}: x6 += b
+  dir{*}: x6 += [null]
+  print $(dir{./}:x6)
+}
+
+x7 = [string] b
+dir{*}: x7 =+ a
+sub/:
+{
+  dir{*}: x7 += c
+  print $(dir{./}:x7)
+}
+
+./:
diff --git a/tests/variable/type-pattern-append/test.out b/tests/variable/type-pattern-append/test.out
new file mode 100644
index 0000000..e8e2242
--- /dev/null
+++ b/tests/variable/type-pattern-append/test.out
@@ -0,0 +1,7 @@
+abc
+b c
+a b
+a b c
+ab
+ab
+abc
diff --git a/tests/variable/type-pattern-append/test.sh b/tests/variable/type-pattern-append/test.sh
new file mode 100755
index 0000000..c745b76
--- /dev/null
+++ b/tests/variable/type-pattern-append/test.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+
+b -q | diff --strip-trailing-cr -u test.out -