Changing default calling convention of c++ sources makes linking fail

[madebr]

When compiling a c++ source with -ecc, linking fails because main_ cannot be found.

In the CMake script below, the a_c executable links succesfully, but a_cxx does not.
Because linking fails, this blocks me from adding -ecc to CMAKE_CXX_FLAGS in CMake toolchain files.

Consider the following cmake script:

set(CMAKE_SYSTEM_NAME "DOS")
set(CMAKE_SYSTEM_PROCESSOR "x86")

set(CMAKE_C_COMPILER "wcl386")
set(CMAKE_CXX_COMPILER "wcl386")

cmake_minimum_required(VERSION 3.20)
project(test C CXX)

file(WRITE a.c [[
int main(int argc, char *argv[]) {
  return 0;
}
]])

file(WRITE a.cpp [[
int main(int argc, char *argv[]) {
  return 0;
}
]])

add_executable(a_c a.c)
target_compile_options(a_c PRIVATE "-ecc")

add_executable(a_cpp a.cpp)
target_compile_options(a_cpp PRIVATE "-ecc")

Complete build log:

$ ninja -j1 -v
[1/4] /home/maarten/owatcom2/binl/wcl386 -zq -d+  -I/home/maarten/owatcom2/h -w3 -bt=dos -d2 -ecc -foCMakeFiles/a_c.dir/a.c.obj -c -cc /tmp/ww/a.c
/tmp/ww/a.c(1): Warning! W303: Parameter 'argv' has been defined, but not referenced
/tmp/ww/a.c(1): Warning! W303: Parameter 'argc' has been defined, but not referenced
[2/4] : && /home/maarten/owatcom2/binl/wlink option quiet name a_c.exe opt map system dos4g debug all file {CMakeFiles/a_c.dir/a.c.obj}   && :
[3/4] /home/maarten/owatcom2/binl/wcl386 -zq -d+  -I/home/maarten/owatcom2/h -w3 -bt=dos -xs -d2 -ecc -foCMakeFiles/a_cpp.dir/a.cpp.obj -c -cc++ /tmp/ww/a.cpp
[4/4] : && /home/maarten/owatcom2/binl/wlink option quiet name a_cpp.exe opt map system dos4g debug all file {CMakeFiles/a_cpp.dir/a.cpp.obj}   && :
FAILED: a_cpp.exe 
: && /home/maarten/owatcom2/binl/wlink option quiet name a_cpp.exe opt map system dos4g debug all file {CMakeFiles/a_cpp.dir/a.cpp.obj}   && :
Error! E2028: main_ is an undefined reference
file clib3r.lib(cmain386.c): undefined symbol main_
ninja: build stopped: subcommand failed.

[jmalak]

see issue #1187 and #1246