bug#34151: [PATCH][bug 34151, 42393, 44795, 49755, 45205, 55073] Fix lex/yacc C++ tests on Solaris/SunOS

[Bogdan]

Hi all.

 The Solaris/SunOS lex generates C++ code with functions properly 
wrapped into >extern "C"<, but only if:

- compiling as C++ (__cplusplus is #defined),

- the feature constant __EXTERN_C__ is #defined (probably meant just 
for the SunOS compiler, not for all).

Other compilers, like GCC/G++, obviously miss the SunOS-specific 
__EXTERN_C__ and thus miss >extern "C"<, declaring the yylex() 
function as C++ (the default). This doesn't need to be a problem, but:

- the function should be declared as the user code expects it (as C 
for C programs and as C++ for C++ programs),

- the Automake lex tests for C++ re-declare the function always as a C 
function.

 The re-declarations happens probably for good reason, like for lex 
implementations which don't declare the function in advance combined 
with C/C++ compilers which always require declarations. Unfortunately, 
having the same function declared as C++ (default) first and then 
later as C causes compiler errors.

 The attached patch corrects the declarations in the following way:

- if we're in C++ mode and __EXTERN_C__ is declared (so, we're on a 
SunOS with their compiler), declare the function as C in C++ mode 
(i.e., use >extern "C"<), just like the SunOS lex does,

- if we're in C++ mode and __EXTERN_C__ is NOT declared, but also 
"__sun" is not declared (so, we're NOT on a SunOS), still assume C++ 
mode, but a different system and declare the function as C in C++ mode 
(i.e., use >extern "C"<) to support other compilers,

- if we're in C++ mode and __EXTERN_C__ is NOT declared, but also 
"__sun" IS declared (so, we ARE on a SunOS, but not using their 
compiler), declare the function as C++ in C++ mode, just like the 
SunOS lex does,

- if we're NOT in C++ mode, declare the function as C. Not the case in 
these tests, though.

 This should match what the SunOS lex/compiler do and expect which 
not braking other systems (re-checked on my Linux - works).

 Verified that the defect exists on a Solaris 11.4 Build 15 (SunOS 
5.11, GCC 7.3.0, amd64) and that the patch fixes it.

 Furthermore, I checked how GCC manages the declarations of standard 
C functions in C++. Seems so simple as saying:

        using std::some_c_library_function_name;

 which the SunOS library (or maybe just the old GCC library there) is 
not doing.
 So, it just takes this to fix the missing prototypes in C++ mode.

 In short, the patch:
- fixes the remaining part of 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=34151,
- fixes the remaining part of 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42393 
(t/silent-many-languages.sh works for me),
- fixes https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44795,
- fixes https://debbugs.gnu.org/cgi/bugreport.cgi?bug=49755,
- partially fixes https://debbugs.gnu.org/cgi/bugreport.cgi?bug=45205,
- partially fixes https://debbugs.gnu.org/cgi/bugreport.cgi?bug=55073.

 Fortunately, these parts are just problems with the tests 
themselves, not with Automake in general.

--
Regards - Bogdan ('bogdro') D.                 (GNU/Linux & FreeDOS)
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