extern "C" { double bleft(double); }which declares sqrt to be a function that takes a double as an argument and returns a double. This information is needed so that C++ can check that your uses of this function have the correct type.
If you're using makefiles, you can have the files combined automatically. You'd could use a Makefile like:
SRCS = foo.C bar.c OBJS = foo.o bar.o .SUFFIXES: .C .o .C.o: ; $(C++) $(C++FLAGS) -c $< C++=g++ prog: $(OBJS) $(C++) $(C++FLAGS) -o prog $(OBJS) -lmto link together foo.c (the c++ file) and bar.c (ordinary C) to make the executable program prog.