HS 2.0R2: Allow custom libcurl linkage for hs20-osu-client

In case someone is compiling their own libcurl and wants to link it
statically, for instance, the new CUST_CURL_LINKAGE parameter can be
used to override the default -lcurl argument.

Signed-off-by: Ben Greear <greearb@candelatech.com>

hs20/client/Makefile

@@ -67,7 +67,13 @@ OBJS += ../../src/crypto/sha256-internal.o
 
 CFLAGS += $(shell xml2-config --cflags)
 LIBS += $(shell xml2-config --libs)
+
+# Allow static/custom linking of libcurl.
+ifdef CUST_CURL_LINKAGE
+LIBS += ${CUST_CURL_LINKAGE}
+else
 LIBS += -lcurl
+endif
 
 CFLAGS += -DEAP_TLS_OPENSSL
 LIBS += -lssl -lcrypto