Broken out-of-tree build

[GitHub issues - opened]

alsa-project/alsa-lib issue #136 was opened from concatime:

Hi.
Due to missing headers, I cannot build alsa-lib if the build directory is outside of the source tree.
```
make[2]: Entering directory '/home/iemaghni/Code/tmp/build/src/ucm'
  CC       utils.lo
In file included from ../../../alsa-lib/src/ucm/ucm_local.h:41,
                 from ../../../alsa-lib/src/ucm/utils.c:33:
../../../alsa-lib/include/use-case.h:45:10: fatal error: alsa/asoundlib.h: No such file or directory
   45 | #include <alsa/asoundlib.h>
      |          ^~~~~~~~~~~~~~~~~~
```
Step to reproduce:
```sh
git clone https://github.com/alsa-project/alsa-lib.git
mkdir build
cd alsa-lib
libtoolize --force --copy --automake
aclocal
autoheader
automake --foreign --copy --add-missing
autoconf
cd ../build
../alsa-lib/configure
make
```
Thank you!

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/136
Repository URL: https://github.com/alsa-project/alsa-lib