libcxx: Add package

Currently in OpenWrt, there are two libc++: libstdcpp and uClibc++. The
former is huge and the latter supports only C++98 with some basic support
for C++11. Those C++ versions seem to be specific to the compiler version

libcxx supports C++11 and above while being much smaller than libstdcpp.
On mt7621, these are the sizes of the ipks that I get:

libstdcpp: 460786
libcxx: 182881
uClibc++:67720

libcxx is faster than uClibc++ and is under active development as part of
the LLVM project while uClibc++ is effectively dead.

This PR modifies uclibc++.mk to expose the make menuconfig option. Further
cleanup is beyond the scope of this PR. What that means is, this is not
used by default.

A g++-libcxx wrapper based on the uClibc++ one was added. Works the same
way.

Compile tested with all packages that use uclibc++.mk in their Makefiles
under mipsel_24kc. kismet fails compilation but that package needs to be
cleaned up and updated.

Runtime tested with gddrescue, gdisk, dcwapd, bonnie++, and aircrack-ng
on a TP-Link Archer C7v2.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

config/Config-build.in

@@ -197,6 +197,9 @@ menu "Global build settings"
 		config USE_UCLIBCXX
 			bool "uClibc++"
 
+		config USE_LIBCXX
+			bool "libc++"
+
 		config USE_LIBSTDCXX
 			bool "libstdc++"
 	endchoice

include/uclibc++.mk

@@ -4,8 +4,8 @@ ifndef DUMP
   endif
 endif
 
-PKG_PREPARED_DEPENDS += CONFIG_USE_UCLIBCXX
-CXX_DEPENDS = +USE_UCLIBCXX:uclibcxx +USE_LIBSTDCXX:libstdcpp
+PKG_PREPARED_DEPENDS += CONFIG_USE_UCLIBCXX CONFIG_USE_LIBCXX
+CXX_DEPENDS = +USE_UCLIBCXX:uclibcxx +USE_LIBCXX:libcxx +USE_LIBSTDCXX:libstdcpp
 
 ifneq ($(CONFIG_USE_UCLIBCXX),)
  ifneq ($(CONFIG_CCACHE),)
@@ -14,3 +14,11 @@ ifneq ($(CONFIG_USE_UCLIBCXX),)
   TARGET_CXX=g++-uc
  endif
 endif
+
+ifneq ($(CONFIG_USE_LIBCXX),)
+ ifneq ($(CONFIG_CCACHE),)
+  TARGET_CXX_NOCACHE=g++-libcxx
+ else
+  TARGET_CXX=g++-libcxx
+ endif
+endif

package/libs/libcxx/Makefile

@@ -0,0 +1,62 @@
+#
+# This is free software, licensed under the GNU General Public License v2.
+# See /LICENSE for more information.
+#
+
+include $(TOPDIR)/rules.mk
+
+PKG_NAME:=libcxx
+PKG_VERSION:=9.0.1
+PKG_RELEASE:=1
+
+PKG_SOURCE_PROTO:=git
+PKG_SOURCE_URL:=https://git.llvm.org/git/libcxx
+PKG_SOURCE_VERSION:=2076f539f410805ef88692b9c0ce0a0b882a7680
+PKG_MIRROR_HASH:=6dff036660d478bfaa14e407fc5de26d22da1087118c897b1a3ad2e90cb7bf39
+
+PKG_MAINTAINER:=Rosen Penev <rosenp@gmail.com>
+PKG_LICENSE:=MIT
+PKG_LICENSE_FILES:=LICENSE.txt
+
+PKG_BUILD_PARALLEL:=1
+CMAKE_BINARY_SUBDIR:=build
+
+include $(INCLUDE_DIR)/package.mk
+include $(INCLUDE_DIR)/cmake.mk
+
+define Package/libcxx
+  SECTION:=libs
+  CATEGORY:=Libraries
+  TITLE:=LLVM libstdc++
+  URL:=https://libcxx.llvm.org/
+  DEPENDS:=+libatomic +libpthread
+endef
+
+define Package/libcxx/description
+  LLVM C++ library targeting C++11 and above
+endef
+
+CMAKE_OPTIONS += \
+	-DLIBCXX_ENABLE_ASSERTIONS=OFF \
+	-DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF \
+	-DLIBCXX_INCLUDE_BENCHMARKS=OFF \
+	-DLIBCXX_INCLUDE_DOCS=OFF \
+	-DLIBCXX_INCLUDE_TESTS=OFF \
+	-DLIBCXX_HAS_MUSL_LIBC=ON \
+	-DLIBCXX_STANDALONE_BUILD=ON
+
+TARGET_CXXFLAGS += -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -Wno-attributes -flto
+TARGET_LDFLAGS += -Wl,--as-needed
+
+define Build/InstallDev
+	$(call Build/InstallDev/cmake,$(1))
+	$(CP) files/g++-libcxx	$(TOOLCHAIN_DIR)/bin/
+	$(SED) 's,CXX,$(TARGET_CXX),g' $(TOOLCHAIN_DIR)/bin/g++-libcxx
+endef
+
+define Package/libcxx/install
+	$(INSTALL_DIR)					$(1)/usr/lib
+	$(CP) $(PKG_INSTALL_DIR)/usr/lib/libc++.so.*	$(1)/usr/lib/
+endef
+
+$(eval $(call BuildPackage,libcxx))

package/libs/libcxx/files/g++-libcxx

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+WRAPPER_INCLUDEDIR="-I$STAGING_DIR/usr/include/c++/v1"
+WRAPPER_LIBDIR="-L$STAGING_DIR/usr/lib"
+WRAPPER_LIBS="-lc -lgcc_s -lssp_nonshared"
+
+WRAPPER_OPTIONS=""
+WRAPPER_INCLIB="Y"
+for arg
+do
+	case "$arg" in
+	-c|-E|-S) WRAPPER_INCLIB="N" ;;
+	-static) [ "$WRAPPER_LIBS" != "-lc -lgcc_s -lssp_nonshared -lgcc_eh" ] && WRAPPER_LIBS="-lc -lgcc_s -lssp_nonshared -lgcc_eh" ;;
+	esac
+done
+[ "$WRAPPER_INCLIB" = "Y" ] && WRAPPER_OPTIONS="-nodefaultlibs $WRAPPER_LIBDIR -lc++ -lsupc++ $WRAPPER_LIBS"
+
+exec CXX -fno-builtin -nostdinc++ -DGCC_HASCLASSVISIBILITY "$WRAPPER_INCLUDEDIR" "$@" $WRAPPER_OPTIONS
+