add support to build with MSVC /Gr (fastcall mode for x86)

not enabled by default nor exposed at the moment

src/shell/datalog_frontend.cpp

@@ -109,7 +109,7 @@ static void on_timeout() {
     exit(0);
 }
 
-static void on_ctrl_c(int) {
+static void STD_CALL on_ctrl_c(int) {
     signal (SIGINT, SIG_DFL);
     display_statistics();
     raise(SIGINT);

src/shell/dimacs_frontend.cpp

@@ -47,7 +47,7 @@ static void on_timeout() {
     exit(0);
 }
 
-static void on_ctrl_c(int) {
+static void STD_CALL on_ctrl_c(int) {
     signal (SIGINT, SIG_DFL);
     display_statistics();
     raise(SIGINT);

src/shell/main.cpp

@@ -291,7 +291,7 @@ char const * get_extension(char const * file_name) {
     }
 }
 
-int main(int argc, char ** argv) {
+int STD_CALL main(int argc, char ** argv) {
     try{
         unsigned return_value = 0;
         memory::initialize(0);

src/shell/opt_frontend.cpp

@@ -305,7 +305,7 @@ static void display_statistics() {
     display_results();
 }
 
-static void on_ctrl_c(int) {
+static void STD_CALL on_ctrl_c(int) {
     if (g_opt && g_first_interrupt) {
         g_opt->get_manager().limit().cancel();
         g_first_interrupt = false;

src/shell/smtlib_frontend.cpp

@@ -62,7 +62,7 @@ static void on_timeout() {
     }
 }
 
-static void on_ctrl_c(int) {
+static void STD_CALL on_ctrl_c(int) {
     signal (SIGINT, SIG_DFL);
     #pragma omp critical (g_display_stats)
     {