ensure CXX is used to link the final shared object

[paleolimbot]

This PR fixes #94, which is a similar problem to one we see in 'arrow' when compiling with slightly different C++ flags (in our case, -stdlib=libc++ to replicate the custom clang build that exists on fedora-clang-devel; see https://github.com/apache/arrow/pull/12734). The final .so is getting linked with CC instead of CXX because there are no .cpp files (I think). This PR just renames init.c to init.cpp and includes the relevant changes to make sure that compile.c/compile.h work as they did before.

With reprex:

# docker pull rhub/fedora-clang-devel
/opt/R-devel/bin/R -e 'install.packages(c("withr", "remotes"), repos = "https://cloud.r-project.org/")'
/opt/R-devel/bin/R -e 'withr::with_makevars(list(CXX = "-stdlib=libc++"), remotes::install_github("rstudio/sass"), assignment = "+=")'
# ...
# ** testing if installed package can be loaded from temporary location
# Error: package or namespace load failed for ‘sass’ in dyn.load(file, DLLpath = DLLpath, ...):
#  unable to load shared object '/opt/R-devel/lib64/R/library/00LOCK-sass/00new/sass/libs/sass.so':
#   /opt/R-devel/lib64/R/library/00LOCK-sass/00new/sass/libs/sass.so: undefined symbol: _ZTINSt3__113basic_ostreamIcNS_11char_traitsIcEEEE
/opt/R-devel/bin/R -e 'withr::with_makevars(list(CXX = "-stdlib=libc++"), remotes::install_github("rstudio/sass#104"), assignment = "+=")'
# ...works!

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[paleolimbot]

(I can't seem to sign the CLA, I just get a window with a select box that won't let me choose anything)

[wch]

Thanks!