autotools setup only allows building with one job at a time when building cpp bindings

[dvzrv]

Describe the bug
When building 19.7.0 as a package for Arch Linux I used the autotools setup and noticed, that the build fails with multiple jobs (our build system automatically sets the jobs for make according to a global MAKEFLAGS (in my case it's -j24).

The build only fails if the C++ bindings are built (i.e. using ./configure --enable-cxx). This issue is also present with 19.6.0.

To be able to build I have to pin the number of jobs to one (i.e. -j1).

To Reproduce
Steps to reproduce the behavior. Include code if applicable.

  cd "${pkgname}-${pkgver}"
  (
  cd bindings/cpp
  autoreconf -fiv
  )
  autoreconf -fiv

  ./configure --prefix=/usr \
              --enable-cxx
  make

Expected behavior
The build works with multiple jobs (e.g. facilitating as many CPU cores as there are available).

Actual behavior
The build fails and does not give a specific error message as to why:
Build log:
portaudio-19.7.0-build.log

Desktop (please complete the following information):

• OS: Arch Linux
• OS Version: n/a
• PortAudio version: 19.7.0
• If Windows or Linux, which Host API (e.g. WASAPI): n/a

Additional context
Being able to build in parallel (with multiple jobs) enables for faster builds.
Concurrency issues in the build usually point at e.g. the same files being used by multiple (sub)targets.

[RossBencina]

We're going to move C++ bindings to their own repo #548

[dvzrv]

@RossBencina cool! Please feel free to move the issues relevant for them to that repository then (unless the move fixes the build system related issues). Thanks! :)