Using Static Library instead of Shared Library
The problem is not with the libray, but with the include files that come with it. Basically some of your source files need to use the curl/curl.h header file, and the compiler cannot find it. You'll need to add back the -I/usr/local/lib argument to CFLAGS, as the -I argument tells the compiler about additional directories where it can search for header files:
CFLAGS += -Bstatic -L. -lcurl -I/usr/local/lib