Fix linker error when configured with --enable-lcov

[droark]

Tested on OS X and Linux. Haven't tested on Windows. Can somebody check and see if this is necessary over there? I assume it is.

[gmaxwell]

I think this should be passing --coverage and not -fprofile-arcs as an ldflag (the actual behavior we were missing in the linker -lgcov but --coverage is an alias for "do the right thing for coverage"). Apparently -fprofile-arcs is also doing the same thing right now for linking, but I wouldn't depend on it to be as robust or to pick up future coverage knobs.

I wasn't previously aware we had this option, whenever I previously wanted to generate coverage I'd twiddled the cflags and ldflags myself.

[droark]

@gmaxwell - Yeah, I didn't know either. It's amazing what a code deep dive will turn up. :) I'm tempted to write a doc with all the config options and PR it.

Anyway, you're right. I somehow missed the lcov web page, which says to use --coverage for both the compiler and linker. Just compiled and linked on OSX (make check passes too), and did a fixup.

[gmaxwell]

ACK

[sipa]

utACK

[btcdrak]

utACK

[jonasschnelli]

utACK e2b5c98