glog: disable on blackfin external toolchains

glog: disable on blackfin external toolchains

With following blackfin toolchains

BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX_2014R1
BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX_2013R1
BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX_2012R2

when BR2_BINFMT_FDPIC=y

../usr/include/bits/elf-fdpic.h: In function 'void* __reloc_pointer(void*, const elf32_fdpic_loadmap*)':
../usr/include/bits/elf-fdpic.h:95: error: invalid use of 'void'

code snippet at line 95 in elf-fdpic.h
-------------------------------------------------------
unsigned long offset = p - (void*)map->segs[c].p_vaddr;
-------------------------------------------------------

void pointer addition and subtraction is not allowed in C++ as it has undetermined size,
however in C with language extension it is possible because sizeof void is treated
as one byte.

This is toolchain-specific issue. So disable package on these external toolchains.

fixes:
http://autobuild.buildroot.net/results/c70/c704c70ae2f066f85dd6a5fa6a73789bc358d368/

Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>