libressl-cmake.sh: fix to initialize CFLAGS [ci skip]

Regression from c29fb63

Author: vszakats

libressl-cmake.sh

@@ -16,6 +16,8 @@ _VER="$1"
 
   rm -r -f "${_PKGDIR:?}" "${_BLDDIR:?}"
 
+  CFLAGS=''
+
   if [ "${_CC}" = 'llvm' ]; then
     CFLAGS="${CFLAGS} -Wa,--noexecstack"
   fi