Allow -g and -O2 as the arguments of config/xclang.
Test aligned_alloc or posix_memalign with AC_LINK_IFELSE.

AC_COMPILE_IFELSE() won't work because C89 allows the
implicit declartion of functions.  We should test these
functions by linking the code with libc.
Move the #endif in pocl_utils.c.

It seems that #if on line 191 should only apply to
pocl_aligned_free(), thus we have to move the #endif.
Otherwise, pocl_create_event() and pocl_create_command()
won't be compiled and will result in link error.
Merge branch 'fix-memalign-check' of github.com:loganchien/pocl