update to c11, fix kul-optec#34

to fix build error:
```
gcc -g -Wall -std=c99 -Wwrite-strings -funroll-loops -Wstrict-prototypes -I. -Iinclude -fPIC -Ofast -DSVD_ACTIVATED=1 -DCTRLC=1  -DCOPYAMATRIX=1  -DLAPACK_LIB_FOUND -DUSE_LAPACK -c src/scs.c -o out/obj/scs.o
In file included from include/scs.h:36,
                 from src/scs.c:28:
include/util.h:78:25: error: field 'tic' has incomplete type
   78 |         struct timespec tic;
      |                         ^~~
include/util.h:79:25: error: field 'toc' has incomplete type
   79 |         struct timespec toc;
      |                         ^~~
make: *** [Makefile:59: out/obj/scs.o] Error 1
```

python/setup.py

@@ -182,7 +182,7 @@ def install_scs(**kwargs):
     blas_info = kwargs['blas_info']
     lapack_info = kwargs['lapack_info']
 
-    extra_compile_args = ["-O3 -std=c99"]
+    extra_compile_args = ["-O3"]
     library_dirs = []
     extra_link_args = []
     libraries = []

scs.mk

@@ -50,7 +50,7 @@ endif
 endif
 
 # Add on default CFLAGS
-CFLAGS += -g -Wall -std=c99 -Wwrite-strings -funroll-loops -Wstrict-prototypes -I. -Iinclude
+CFLAGS += -g -Wall -Wwrite-strings -funroll-loops -Wstrict-prototypes -I. -Iinclude
 ifneq ($(ISWINDOWS), 1)
 CFLAGS += -fPIC
 endif