Previously, when parsing linker parameters for C++ targets, the CC
variable was used to determine what the "prefix" of the command was in
order to determine what the linker arguments were.

If the value of LDCXXSHARED did not match CC, the first argument would
be dropped as it was assumed to be the linker command.

However, if the command was a wrapper, such as ccache, it could lead to
compile problems as the generated command would be incorrect.

In the following scenario:
  LDCXXSHARED="ccache g++ -shared -Wl,--enable-new-dtags"
  CC="ccache gcc"
  CXX="ccache g++"

The command would be incorrectly parsed to:
  ccache g++ g++ -shared -Wl,--enable-new-dtags

Now, the CXX value is used to improve the chances of parsing the linker
arguments correctly to generate:
  ccache g++ -shared -Wl,--enable-new-dtags

LDCXXSHARED and CXX still need to be in sync either in the environment
or within the sysconfig variables in the CPython build for parsing to
work correctly.

The CXX value is now also respected when linking executable binaries.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

Address the following:

  * use the compiler's executable extension in asserts
  * remove any environment variables that may have been injected by CI
  * add a pragma to ignore a line without coverage

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

…arg-parse

Respect CXX when parsing linker parameters for UNIX c++ targets.

Closes pypa/distutils#355