add a debug argument to load_all()

[mcol]

I've spent quite some time trying to understand why, when compiling some C++ code with devtools::load_all(), I would always get the g -O0 flags (among others) at the very end, thus overriding my preferred settings in ~/.R/Makevars or src/Makevars. Eventually I've worked out that these are appended by pkgbuild::compile_dll(), which is called by pkgload::load_all().

I think the current default makes sense, but it would be nice if pkgload::load_all() had a debug flag that gets passed to pkgbuild::compile_dll(), so that the debug options could be disabled easily. Alternatively, load_all() could accept a ... argument and pass that to compile_dll() (this seems to me an inferior solution though).

[gaborcsardi]

FWIW you can also set the pkg.build_extra_flags option or the PKG_BUILD_EXTRA_FLAGS env var for pkgbuild, see https://github.com/r-lib/pkgbuild#options