Mark R_init_xml2 as visible

Closes #451
Closes #450

Author: jeroen

src/Makevars.in

@@ -1,3 +1,4 @@
 PKG_CPPFLAGS=-I../inst/include @cflags@ -DUCHAR_TYPE=wchar_t -DU_SHOW_CPLUSPLUS_API=0 -DSTRICT_R_HEADERS -DR_NO_REMAP
-PKG_CXXFLAGS=$(C_VISIBILITY)
+PKG_CFLAGS=$(C_VISIBILITY)
+PKG_CXXFLAGS=$(CXX_VISIBILITY)
 PKG_LIBS=@libs@

src/init.c

@@ -1,6 +1,7 @@
 #include <Rinternals.h>
 #include <stdlib.h> // for NULL
 #include <R_ext/Rdynload.h>
+#include <R_ext/Visibility.h>
 
 /* FIXME:
    Check these declarations against the C/Fortran source code.
@@ -137,8 +138,7 @@ static const R_CallMethodDef CallEntries[] = {
 };
 
 void init_libxml2_library(void);
-void R_init_xml2(DllInfo *dll)
-{
+attribute_visible void R_init_xml2(DllInfo *dll) {
     R_registerRoutines(dll, NULL, CallEntries, NULL, NULL);
     R_useDynamicSymbols(dll, FALSE);
     init_libxml2_library();