Compiling C programs
n.o is made automatically from n.c with a recipe of the form ‘$(CC) $(CPPFLAGS) $(CFLAGS) -c’.

Compiling C++ programs
n.o is made automatically from n.cc, n.cpp, or n.C with a recipe of the form ‘$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c’. 
We encourage you to use the suffix ‘.cc’ for C++ source files instead of ‘.C’.

Linking a single object file
n is made automatically from n.o by running the linker (usually called ld) via the C compiler. The precise recipe 
used is ‘$(CC) $(LDFLAGS) n.o $(LOADLIBES) $(LDLIBS)’.

This rule does the right thing for a simple program with only one source file. It will also do the right thing 
if there are multiple object files (presumably coming from various other source files), one of which has a name 
matching that of the executable file. Thus,

x: y.o z.o
when x.c, y.c and z.c all exist will execute:

cc -c x.c -o x.o
cc -c y.c -o y.o
cc -c z.c -o z.o
cc x.o y.o z.o -o x
rm -f x.o
rm -f y.o
rm -f z.o
In more complicated cases, such as when there is no object file whose name derives from the executable file name, 
you must write an explicit recipe for linking.

CFLAGS
Extra flags to give to the C compiler.

CXXFLAGS
Extra flags to give to the C++ compiler.

CPPFLAGS
Extra flags to give to the C preprocessor and programs that use it (the C and Fortran compilers).

LDFLAGS
Extra flags to give to compilers when they are supposed to invoke the linker, ‘ld’, such as -L. 
Libraries (-lfoo) should be added to the LDLIBS variable instead.

LDLIBS
Library flags or names given to compilers when they are supposed to invoke the linker, ‘ld’. 
LOADLIBES is a deprecated (but still supported) alternative to LDLIBS. Non-library linker flags, 
such as -L, should go in the LDFLAGS variable.

%.o : %.s