R: sanitize Makeconf flags

Closes #4692.

srcpkgs/R/template

@@ -1,7 +1,7 @@
 # Template file for 'R'
 pkgname=R
 version=3.3.1
-revision=1
+revision=2
 nocross=yes
 build_style=gnu-configure
 configure_args="--docdir=/usr/share/doc/R rdocdir=/usr/share/doc/R
@@ -26,3 +26,10 @@ pre_configure() {
 	export R_PDFVIEWER=/usr/bin/xdg-open
 	export FFLAGS="$CFLAGS"
 }
+
+post_install() {
+	# Sanitize Makeconf flags
+	sed -i -e 's/^\(CFLAGS\|CXXFLAGS\|CXX1XFLAGS\|FFLAGS\|SAFE_FFLAGS\) = .*/\1 = -O2 -g $(LTO)/' \
+		-e 's/^LDFLAGS = .*/LDFLAGS =/' \
+		$DESTDIR/usr/lib/R/etc/Makeconf
+}