Install internal headers for Go bindings

Go, and likely other language bindings will, need access to create
new combinators. This functionality is currently located
src/parsers/parser_internal.h. Install this header to the system with
dependent headers until a better solution can be found.

src/SConscript

@@ -6,7 +6,17 @@ bindings = []
 dist_headers = [
     "hammer.h",
     "allocator.h",
-    "glue.h"
+    "glue.h",
+    "internal.h"
+]
+
+parsers_headers = [
+    "parsers/parser_internal.h"
+]
+
+backends_headers = [
+    "backends/regex.h",
+    "backends/contextfree.h"
 ]
 
 parsers = ['parsers/%s.c'%s for s in
@@ -64,6 +74,8 @@ libhammer_static = env.StaticLibrary('hammer', parsers + backends + misc_hammer_
 
 env.Install("$libpath", [libhammer_static, libhammer_shared])
 env.Install("$incpath", dist_headers)
+env.Install("$parsersincpath", parsers_headers)
+env.Install("$backendsincpath", backends_headers)
 env.Install("$pkgconfigpath", "../../../libhammer.pc")
 
 testenv = env.Clone()