Add operation support in enum

With this patch, the following C code works now:

```c
typedef enum VkPipelineCacheHeaderVersion {
    VK_PIPELINE_CACHE_HEADER_VERSION_ONE = 1,
    VK_PIPELINE_CACHE_HEADER_VERSION_RANGE_SIZE = (VK_PIPELINE_CACHE_HEADER_VERSION_ONE - VK_PIPELINE_CACHE_HEADER_VERSION_ONE + 1),
} VkPipelineCacheHeaderVersion;
```

1 files changed, 10 insertions, 0 deletions

diff --git a/cffi/cparser.py b/cffi/cparser.py
index 3d5caed..0c8ef3f 100644
--- a/cffi/cparser.py
+++ b/cffi/cparser.py
@@ -803,6 +803,16 @@ class Parser(object):
                            "the actual array length in this context"
                            % exprnode.coord.line)
         #
+        if (isinstance(exprnode, pycparser.c_ast.BinaryOp) and
+                exprnode.op == '+'):
+            return (self._parse_constant(exprnode.left) +
+                    self._parse_constant(exprnode.right))
+        #
+        if (isinstance(exprnode, pycparser.c_ast.BinaryOp) and
+                exprnode.op == '-'):
+            return (self._parse_constant(exprnode.left) -
+                    self._parse_constant(exprnode.right))
+        #
         raise FFIError(":%d: unsupported expression: expected a "
                        "simple numeric constant" % exprnode.coord.line)