Prefer using PKG_CPPFLAGS for ragg headers
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From https://cran.r-project.org/doc/manuals/R-exts.html#Using-Makevars:
N.B.: Include paths are preprocessor options, not compiler options, and must be set in PKG_CPPFLAGS as otherwise platform-specific paths (e.g. ‘-I/usr/local/include’) will take precedence. PKG_CPPFLAGS should contain ‘-I’, ‘-D’, ‘-U’ and (where supported) ‘-include’ and ‘-pthread’ options: everything else should be a compiler flag. The order of flags matters, and using ‘-I’ in PKG_CFLAGS or PKG_CXXFLAGS has led to hard-to-debug platform-specific errors.
And also:
Use #include "my.h" not #include <my.h> for headers in your package. The second form is intended for system headers and the search order for such headers is platform-dependent (and may not include the current directory). For extra safety, name headers in a way that cannot be confused with a system header so not, for example, types.h.
Because Makevars.in sets the include path via PKG_CXXFLAGS here:
Line 1 in f6acc4c
we run the risk of picking up an alternate installation of agg, rather than the bundled version. It should suffice to just set the include paths in PKG_CPPFLAGS rather than updating how agg is included, though.