replace gcc with clang to avoid errors from gcc 7.4/8.1 compiler and ld linker

[liuqun]

1. make CC=clang success

   liuqun@vmware:~/aaa/dpdk/lstack$ make clean
   rm -f main main.o lstack.o sha1.o
   liuqun@vmware:~/aaa/dpdk/lstack$ make CC=clang
   clang -std=c11 -Wall -Wextra -O3 -mcx16   -c -o main.o main.c
   clang -std=c11 -Wall -Wextra -O3 -mcx16   -c -o lstack.o lstack.c
   clang -std=c11 -Wall -Wextra -O3 -mcx16   -c -o sha1.o sha1.c
   clang -std=c11 -Wall -Wextra -O3 -mcx16 -pthread -o main main.o lstack.o sha1.o 

2. GCC-7.4 and GCC-8 with /usr/bin/ld error message:

   liuqun@vmware:~/aaa/dpdk/lstack$ make clean
   rm -f main main.o lstack.o sha1.o
   liuqun@vmware:~/aaa/dpdk/lstack$ make CC=gcc-7
   gcc-7 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o main.o main.c
   gcc-7 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o lstack.o lstack.c
   gcc-7 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o sha1.o sha1.c
   gcc-7 -std=c11 -Wall -Wextra -O3 -mcx16 -pthread -o main main.o lstack.o sha1.o 
   /usr/bin/ld: lstack.o: in function `lstack_init':
   lstack.c:(.text+0x1b): undefined reference to `__atomic_store_16'
   /usr/bin/ld: lstack.c:(.text+0x75): undefined reference to `__atomic_store_16'
   /usr/bin/ld: lstack.o: in function `lstack_push':
   lstack.c:(.text+0xc0): undefined reference to `__atomic_load_16'
   /usr/bin/ld: lstack.c:(.text+0xf5): undefined reference to `__atomic_compare_exchange_16'
   /usr/bin/ld: lstack.c:(.text+0x121): undefined reference to `__atomic_load_16'
   /usr/bin/ld: lstack.c:(.text+0x14f): undefined reference to `__atomic_compare_exchange_16'
   /usr/bin/ld: lstack.o: in function `lstack_pop':
   lstack.c:(.text+0x1dd): undefined reference to `__atomic_load_16'
   /usr/bin/ld: lstack.c:(.text+0x212): undefined reference to `__atomic_compare_exchange_16'
   /usr/bin/ld: lstack.c:(.text+0x23b): undefined reference to `__atomic_load_16'
   /usr/bin/ld: lstack.c:(.text+0x269): undefined reference to `__atomic_compare_exchange_16'
   collect2: error: ld returned 1 exit status
   make: *** [Makefile:9：main] Error 1

liuqun@vmware:~/aaa/dpdk/lstack$ make clean
rm -f main main.o lstack.o sha1.o
liuqun@vmware:~/aaa/dpdk/lstack$ make CC=gcc-8
gcc-8 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o main.o main.c
gcc-8 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o lstack.o lstack.c
gcc-8 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o sha1.o sha1.c
gcc-8 -std=c11 -Wall -Wextra -O3 -mcx16 -pthread -o main main.o lstack.o sha1.o 
/usr/bin/ld: lstack.o: in function `lstack_init':
lstack.c:(.text+0x23): undefined reference to `__atomic_store_16'
/usr/bin/ld: lstack.c:(.text+0xf4): undefined reference to `__atomic_store_16'
/usr/bin/ld: lstack.o: in function `lstack_push':
lstack.c:(.text+0x14d): undefined reference to `__atomic_load_16'
/usr/bin/ld: lstack.c:(.text+0x185): undefined reference to `__atomic_compare_exchange_16'
/usr/bin/ld: lstack.c:(.text+0x1a8): undefined reference to `__atomic_load_16'
/usr/bin/ld: lstack.c:(.text+0x1d6): undefined reference to `__atomic_compare_exchange_16'
/usr/bin/ld: lstack.o: in function `lstack_pop':
lstack.c:(.text+0x25a): undefined reference to `__atomic_load_16'
/usr/bin/ld: lstack.c:(.text+0x292): undefined reference to `__atomic_compare_exchange_16'
/usr/bin/ld: lstack.c:(.text+0x2bb): undefined reference to `__atomic_load_16'
/usr/bin/ld: lstack.c:(.text+0x2e9): undefined reference to `__atomic_compare_exchange_16'
collect2: error: ld returned 1 exit status
make: *** [Makefile:9：main] Error 1
liuqun@vmware:~/aaa/dpdk/lstack$ 

3. GCC-6 worked for me

   liuqun@vmware:~/aaa/dpdk/lstack$ make clean
   rm -f main main.o lstack.o sha1.o
   liuqun@vmware:~/aaa/dpdk/lstack$ make CC=gcc-6
   gcc-6 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o main.o main.c
   gcc-6 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o lstack.o lstack.c
   gcc-6 -std=c11 -Wall -Wextra -O3 -mcx16   -c -o sha1.o sha1.c
   gcc-6 -std=c11 -Wall -Wextra -O3 -mcx16 -pthread -o main main.o lstack.o sha1.o 

[skeeto]

No need to switch specifically to Clang. Looks like GCC now explicitly requires linking against libatomic, so it just needs another compiler flag. It's pretty irritating when the C / POSIX standard library is spread out such that users must know its structure for each particular system (-lm, -lsocket, -lnsl, -lrt, and now -latomic) but whatever.