Advertise obstack only when chosen
This is similar to this patch for GNU glob:
ff6ce3999db93a52d1cd22510f8508b660ddf54e.
The problem is that libiberty/obstack.c file checks for
_GNU_OBSTACK_INTERFACE_VERSION == _OBSTACK_INTERFACE_VERSION and elides
libiberty obstack implementation if there is already a compatible
implementation in libc. Therefore define should be set only when obstack is
actually selected, otherwise it is not possible to build binutils and gdb
with uClibc.
Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>