Add support for target and prerequisite specific variable blocks

For example, now instead of:

lib{foo}: cxx.loptions += -static
lib{foo}: cxx.libs += -lpthread

We can write:

lib{foo}:
{
  cxx.loptions += -static
  cxx.libs += -lpthread
}

The same works for prerequisites as well as target type/patterns. For
example:

exe{*.test}:
{
  test = true
  install = false
}

1 files changed, 34 insertions, 2 deletions

diff --git a/tests/variable/target-specific/testscript b/tests/variable/target-specific/testscript
index 497c863..8b0a846 100644
--- a/tests/variable/target-specific/testscript
+++ b/tests/variable/target-specific/testscript
@@ -1,10 +1,10 @@
-# file      : tests/variable/scope-specific/testscript
+# file      : tests/variable/target-specific/testscript
 # copyright : Copyright (c) 2014-2018 Code Synthesis Ltd
 # license   : MIT; see accompanying LICENSE file
 
 .include ../../common.testscript
 
-: basic-line
+: basic
 :
 $* <<EOI >>EOO
 x = x
@@ -25,6 +25,38 @@ X
 Z
 EOO
 
+: block
+:
+$* <<EOI >>EOO
+x = x
+./:
+{
+  x = X
+  y = $x
+}
+print $(./: x)
+print $(./: y)
+EOI
+X
+X
+EOO
+
+: block-multiple
+:
+$* <<EOI >>EOO
+x = x
+file{foo} file{bar}:
+{
+  x += X
+  y = $x
+}
+print $(file{foo}: y)
+print $(file{bar}: y)
+EOI
+x X
+x X
+EOO
+
 : eval-qual
 :
 $* <<EOI >>EOO