switch to system2 to be able to capture stderr on error (closes #1257)

[Enchufa2]

Minimal changes, fixes #1257. According to rcheology, system2 has been around since 2.12.0, and allows us to capture both stdout and stderr when showOutput=FALSE. A collateral advantage is that now, when an error happens, the output is in the correct order (previously, the error appeared first, then the compilation command).

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[eddelbuettel]

Excellent