use PKG_ prefix for flags; add -std=c++17

recipe/bld.bat

@@ -1,9 +1,19 @@
+@echo on
+
 bash %RECIPE_DIR%/build_win.sh
 IF %ERRORLEVEL% NEQ 0 exit 1
+
 cp %RECIPE_DIR%/configure.win r
 IF %ERRORLEVEL% NEQ 0 exit 1
+
 cp %RECIPE_DIR%/install.libs.R r/src
 IF %ERRORLEVEL% NEQ 0 exit 1
+
+:: translate our default flags for R; set C++17 due to abseil
+set "PKG_CFLAGS=%CFLAGS%"
+set "PKG_CPPFLAGS=%CPPFLAGS%"
+set "PKG_CXXFLAGS=%CXXFLAGS% -std=c++17"
+
 set "MAKEFLAGS=-j%CPU_COUNT%"
 "%R%" CMD INSTALL --build r
 IF %ERRORLEVEL% NEQ 0 exit 1

recipe/build.sh

@@ -1,10 +1,16 @@
 #!/bin/bash
+set -ex
 
 export DISABLE_AUTOBREW=1
 
+# translate our default flags for R; set C++17 due to abseil
+export PKG_CFLAGS="${CFLAGS}"
+export PKG_CPPFLAGS="${CPPFLAGS}"
+export PKG_CXXFLAGS="${CXXFLAGS} -std=c++17"
+
 if [[ "${target_platform}" == osx-* ]]; then
     # See https://conda-forge.org/docs/maintainer/knowledge_base.html#newer-c-features-with-old-sdk
-    CXXFLAGS="${CXXFLAGS} -D_LIBCPP_DISABLE_AVAILABILITY"
+    export PKG_CXXFLAGS="${PKG_CXXFLAGS} -D_LIBCPP_DISABLE_AVAILABILITY"
 fi
 
 # shellcheck disable=SC2086