nlohmannjson: initial commit

JSON for Modern C++ library.
Design goals: intuitive syntax, trivial integration, serious testing.
https://nlohmann.github.io/json/
IPFS C++ HTTP API client library depends on this library.

Signed-off-by: Leonid Esman <leonid.esman@gmail.com>

libs/nlohmannjson/Makefile

@@ -0,0 +1,38 @@
+#
+# This is free software, licensed under the GNU General Public License v2.
+#
+
+include $(TOPDIR)/rules.mk
+
+PKG_NAME:=nlohmannjson
+PKG_VERSION:=3.7.3
+PKG_RELEASE:=1
+
+PKG_MAINTAINER:=Leonid Esman <leonid.esman@gmail.com>
+PKG_LICENSE:=MIT
+PKG_LICENSE_FILES:=LICENSE.MIT
+
+PKG_BUILD_DIR:=$(BUILD_DIR)/json-$(PKG_VERSION)
+PKG_SOURCE:=json-$(PKG_VERSION).zip
+PKG_SOURCE_URL:=https://codeload.github.com/nlohmann/json/zip/v$(PKG_VERSION)?
+PKG_HASH:=e109cd4a9d1d463a62f0a81d7c6719ecd780a52fb80a22b901ed5b6fe43fb45b
+
+include $(INCLUDE_DIR)/package.mk
+
+define Package/nlohmannjson
+  SECTION:=libs
+  CATEGORY:=Libraries
+  TITLE:=JSON for Modern C++
+  URL:=https://nlohmann.github.io/json/
+endef
+
+define Package/nlohmannjson/description
+  Niels Lohmann's JSON headers-only library for modern C++
+endef
+
+define Build/InstallDev
+	$(INSTALL_DIR) $(1)/usr/include
+	$(CP) $(PKG_BUILD_DIR)/include/nlohmann $(1)/usr/include
+endef
+
+$(eval $(call BuildPackage,nlohmannjson))