xlib_libXfont: work around missing __ELF__ symbol on microblaze

Fixes:
http://autobuild.buildroot.net/results/688/688a839ec57e7bc83b490535871b2dac0b1309ff/
http://autobuild.buildroot.net/results/75c/75c676b1baed6f39657578caf7fc213ce71c1089/
http://autobuild.buildroot.net/results/2b9/2b9e9ebaaabe4880e7d258d88f12ce90738ca779/
http://autobuild.buildroot.net/results/988/988022e559af81d9b7dfc716dfb84bd0fa3698f2/



And many more.

The microblaze toolchains don't define the __ELF__ preprocessor symbol even
though they do use the elf format.  LibXfont checks for this symbol to know
if weak symbols are supported, and otherwise falls back to emulation code
using dlopen - Causing linker issues for stuff using libXfont.

Work around it by defining the symbol here as well.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>