- config.h is now search for under PULSE_DIR
- Location of config.h can be specified explicitly with PULSE_CONFIG_DIR
- Extra warning if PULSE_DIR appears to be invalid

as xrdp already uses config_ac.h. To be consistent, let's use
config_ac.h rather than module-config.h.

* name bootstrap script bootstrap as well as other xrdp repos
* use pkg-config to get pulseaudio version and module directories
* guess prefix by checking for pulseaudio executable
* simplify message then PULSE_DIR not specified
  as m4_text_box doesn't support multiple lines

so our modules will be installed where system's pulseaudio modules are.
Unfortunately, pulseaudio don't check `/usr/local` first