Handle linker scripts

[aniederl]

On some systems a .so file can be a linker script referencing the actual library .so file.

For example, /usr/lib/libncurses.so on Gentoo has this content:

/* GNU ld script
   Since Gentoo has critical dynamic libraries in /lib, and the static versions
   in /usr/lib, we need to have a "fake" dynamic lib in /usr/lib, otherwise we
   run into linking problems.  This "fake" dynamic lib is a linker script that
   redirects the linker to the real lib.  And yes, this works in the cross-
   compiling scenario as the sysroot-ed linker will prepend the real path.

   See bug http://bugs.gentoo.org/4411 for more info.
*/
OUTPUT_FORMAT ( elf64-x86-64 )
GROUP ( /lib64/libncurses.so.5 )

dlopen on such a .so file will produce an error:

Could not open library 'libncurses.so': /usr/lib64/libncurses.so: invalid ELF header

This commit parses the dlerror message for an "invalid ELF header" and sees if a
GROUP() command exists in the file.
If found, it tries to load the referenced file instead.