Improve gcc and include g++, clang and clang++

eg/examples/clang++.md

@@ -0,0 +1 @@
+gcc.md

eg/examples/clang.md

@@ -0,0 +1 @@
+gcc.md

eg/examples/g++.md

@@ -0,0 +1 @@
+gcc.md

eg/examples/gcc.md

@@ -1,30 +1,58 @@
 # gcc
 
-compile main.c (default executable output is a.out)
+compile *main.c* (default executable output is *a.out*)
 
     gcc main.c
 
 
-compile main.c into an outfile file main.o
+compile *main.c* into an outfile file main.o
 
     gcc main.c -o main.o
 
 
-compile main.c and enable all warnings
+compile *main.c* and enable all warnings
 
     gcc -Wall main.c
 
 
-compile main.c but do not run the linker
+compile *main.c* but do not run the linker
 
     gcc -c main.c
 
 
+do not compile, only run the linker
+
+    gcc main.o
+
+
+compile with mamimum optimization level
+
+    gcc -O3 main.c
+
+
+compile with size optimization enabled
+
+    gcc -Os main.c
+
+
+assemble *main.c* (default output is *main.s*)
+
+    gcc -S main.c
+
+
 
 # Basic Usage
 
-Compile a file using the GNU compiler: 
+Compile a file using the GNU compiler:
 
     gcc <file>
 
 
+
+# Note
+
+`gcc` can be replaced by `g++` to compile C++ files.
+It can also be replaced by `clang` and `clang++` to use the Clang compiler
+instead, which has the same command line interface.
+
+