pkgbuild 1.0.4

• build() gains a clean_doc argument, to control if the inst/doc
  directory is cleaned before building. (#79, #75)

• build() and pkgbuild_process now have standard output and error are
  correctly interleaved, by redirecting the standard error of build process
  to the standard output (@gaborcsardi, #78).

• check_build_tools() now has a more helpful error message which points you
  towards ways to debug the issue (#68).

• pkgbuild_process now do not set custom compiler flags, and it uses
  the user's Makevars file (@gaborcsardi, #76).

• rtools_path() now returns NA on non-windows systems and also works when
  has_rtools() has not been run previously (#74).