Fix install when using a separated build directory (#29)
Fix install when using a separated build directory (#29)

This fixes the following error at 'make install' time when
using a separated build directory (out-of-tree build):

CMake Error at cmake_install.cmake:50 (file):
  file INSTALL cannot find "/build/libilbc/src/libilbc/ilbc_export.h": No
  such file or directory.
make: *** [Makefile:105: install] Error 1
make: Leaving directory '/build/libilbc/src/build'

How to reproduce:

$ git clone --recursive https://github.com/TimothyGu/libilbc.git
$ cmake -B build -S libilbc
$ make -C build
$ make -C build install

Fixes #28
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