Fix libcurl include path

Fix the include path for libcurl, if it was installed in a location
where gcc does not look by default. The variable is declared in
the LIBCURL_CHECK_CONFIG m4 macro.

Makefile.am

@@ -19,6 +19,7 @@ minerd_SOURCES	= elist.h miner.h compat.h			\
 		  sha256_cryptopp.c sha256_sse2_amd64.c
 minerd_LDFLAGS	= $(PTHREAD_FLAGS)
 minerd_LDADD	= @LIBCURL@ @JANSSON_LIBS@ @PTHREAD_LIBS@
+minerd_CPPFLAGS = @LIBCURL_CPPFLAGS@
 
 if HAVE_x86_64
 if HAS_YASM