Fix configuration checks performed with ldc2 compiler

[holtrop]

The following Rsconscript fails to configure:

configure do
  check_d_compiler "ldc2"
  check_lib "SDL2"
end

rscons produces this config.log:

$ cat build/_configure/config.log 
Command: ldc2 -c -o build/_configure/cfgtest.o build/_configure/cfgtest.d
Exit status: 256
ldc2: Unknown command line argument '-o'.  Try: 'ldc2 --help'
ldc2: Did you mean '-H'?
Command: ldc2 -c -of build/_configure/cfgtest.o build/_configure/cfgtest.d
Exit status: 0
Command: gcc -of build/_configure/cfgtest.exe build/_configure/cfgtest.c -lSDL2
Exit status: 256
/usr/bin/ld: cannot use executable file 'build/_configure/cfgtest.exe' as input to a link
collect2: error: ld returned 1 exit status

rscons passes the ldc2 -of argument to gcc when attempting the link. Investigate if rscons can use ldc2 to do the link for these configuration tests. A similar issue might exist for clang if gcc is not available.