Bug#670566: arduino-mk: Including libraries is broken

[Jérôme]

> -USER_LIB_CPP_SRC   = $(wildcard $(patsubst %,%/*.cpp,$(USER_LIBS)))
> -USER_LIB_C_SRC     = $(wildcard $(patsubst %,%/*.c,$(USER_LIBS)))
> +USER_LIB_CPP_SRCS   = $(wildcard $(patsubst %,%/*.cpp,$(USER_LIBS)))
> +USER_LIB_C_SRCS     = $(wildcard $(patsubst %,%/*.c,$(USER_LIBS)))

FWIW, this is part of a larger commit that came a few days after the commit
introducing the feature (and the typo) :

https://github.com/mjoldfield/Arduino-Makefile/commit/de2b5273ec27fbf418c642a34af4721608cd4163

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Jérôme