build: don't limit C std to c99 (gnu99)

Now that C11 atomics and thread local storage are supported, the
compiler can figure out what version to use.

configure.ac

@@ -6,7 +6,6 @@
     AM_INIT_AUTOMAKE([tar-ustar subdir-objects])
 
     AC_LANG([C])
-    AC_PROG_CC_C99
     LT_INIT
     PKG_PROG_PKG_CONFIG
 

src/Makefile.am

@@ -634,7 +634,6 @@ endif
 # default CFLAGS
 AM_CFLAGS = ${OPTIMIZATION_CFLAGS} ${GCC_CFLAGS} ${CLANG_CFLAGS}            \
     ${SECCFLAGS} ${PCAP_CFLAGS} -DLOCAL_STATE_DIR=\"$(localstatedir)\"      \
-    -std=gnu99                                                              \
     -Wall -Wno-unused-parameter -Wmissing-prototypes -Wmissing-declarations \
     -Wstrict-prototypes -Wwrite-strings -Wbad-function-cast                 \
     -Wformat-security -Wno-format-nonliteral -Wmissing-format-attribute     \