Remove CXX_STD = CXX11

Mainly prompted to do so by CRAN, but since all modern versions of R require at least C++11 nowadays, this should work fine

src/Makevars

@@ -1,4 +1,6 @@
-# CXX11 is required, apparently CXX14 would be better for constexpr support
+# We no longer specify `CXX_STD = CXX11` since modern versions of R require
+# C++11 as a minimum. This also allows more modern compilers to use C++14, which
+# has constexpr benefits.
 
 # -DINSTALL=dummy
 # To keep some problematic code from running on windows if INSTALL isn't defined.
@@ -24,8 +26,6 @@
 # Because of these issues, we instead default to using an uncompressed
 # text version of the database that we ship with tzdb.
 
-CXX_STD = CXX11
-
 PKG_CXXFLAGS = \
 	-I../inst/include \
 	-DINSTALL=dummy \

src/Makevars.win

@@ -1,5 +1,3 @@
-CXX_STD = CXX11
-
 # https://howardhinnant.github.io/date/tz.html
 # "mingw users: -lpthread and -lole32 are required."
 PKG_LIBS = \