C++: fix uses of inline

Sometimes `inline` would be used in *.cc files on symbols that are not
exported (useless but harmless), and sometimes on exported symbols
such as the constructor of syntax_error (harmful: linking fails).

Reported several times, including:

- by Dennis T
  http://lists.gnu.org/archive/html/bug-bison/2016-03/msg00002.html
- by Frank Heckenbach
  https://savannah.gnu.org/patch/?9616

* data/c++.m4 (b4_inline): New: expands to `inline` or nothing.
Use it where appropriate.
* data/lalr1.cc: Use it where appropriate.

* tests/c++.at (Syntax error as exception): Put the scanner in another
compilation unit to exercise the constructor of syntax_error.

tests/c++.at

@@ -633,6 +633,7 @@ AT_BISON_OPTION_PUSHDEFS([%skeleton "lalr1.cc"])
 
 AT_DATA_GRAMMAR([[input.y]],
 [[%skeleton "lalr1.cc"
+%defines
 
 %code
 {
@@ -664,6 +665,17 @@ item:
 
 %%
 
+void
+yy::parser::error (const std::string &m)
+{
+  std::cerr << "error: " << m << '\n';
+}
+]AT_MAIN_DEFINE[
+]])
+
+AT_DATA_SOURCE([input-scan.cc],
+[[#include "input.hh"
+
 int
 yylex (yy::parser::semantic_type *)
 {
@@ -677,16 +689,9 @@ yylex (yy::parser::semantic_type *)
       return res;
   }
 }
-
-void
-yy::parser::error (const std::string &m)
-{
-  std::cerr << "error: " << m << '\n';
-}
-]AT_MAIN_DEFINE[
 ]])
 
-AT_FULL_COMPILE([[input]])
+AT_FULL_COMPILE([[input]], [[scan]])
 
 AT_PARSER_CHECK([[./input]], [[0]], [[]],
 [[error: invalid expression