Forward declarations of extern "C" functions are not generated correctly

[igrr]

Trying to compile the following sketch:

extern "C" void test() {
}

void setup() {
  test();
}

void loop() {
}

Leads to an error:

/Users/igrokhotkov/Documents/Arduino/Bugreports/B1759_ExternC/B1759_ExternC.ino: In function 'void test()':
B1759_ExternC:1: error: previous declaration of 'void test()' with 'C++' linkage
 extern "C" void test() {
      ^
B1759_ExternC:1: error: conflicts with new declaration with 'C' linkage
 extern "C" void test() {
                      ^
exit status 1
previous declaration of 'void test()' with 'C++' linkage

arduino-builder output:
builder_output.txt

ctags output:
ctags_output.txt
Note that function signature looks correct in ctags output:
B1759_ExternC.ino /^extern "C" void test() {$/;" kind:function line:1 signature:() returntype:void

sketch with prototypes added:

#include <Arduino.h>
#line 1
#line 1 "/Users/igrokhotkov/Documents/Arduino/Bugreports/B1759_ExternC/B1759_ExternC.ino"
#line 1 "/Users/igrokhotkov/Documents/Arduino/Bugreports/B1759_ExternC/B1759_ExternC.ino"
void test();
#line 4 "/Users/igrokhotkov/Documents/Arduino/Bugreports/B1759_ExternC/B1759_ExternC.ino"
void setup();
#line 8 "/Users/igrokhotkov/Documents/Arduino/Bugreports/B1759_ExternC/B1759_ExternC.ino"
void loop();
#line 1
extern "C" void test() {
}

void setup() {
  test();
}

void loop() {
}

I'm not sure which version of Arduino introduced this issue, but this certainly worked fine in 1.6.5.

Original issue filed in our repository: esp8266/Arduino#1759

[facchinm]

Still not completely fixed as reported on #128 (declaration over n lines)