libsysprof: abstract subprocess usage

For embedding cases, we need a bit more control here so that the
application can be in control of how the process is spawned.

src/libsysprof/meson.build

@@ -21,6 +21,7 @@ libsysprof_public_sources = [
   'sysprof-proxy-source.c',
   'sysprof-selection.c',
   'sysprof-source.c',
+  'sysprof-spawnable.c',
   'sysprof-symbol-dirs.c',
   'sysprof-symbol-resolver.c',
   'sysprof-symbols-source.c',
@@ -46,6 +47,7 @@ libsysprof_public_headers = [
   'sysprof-proxy-source.h',
   'sysprof-selection.h',
   'sysprof-source.h',
+  'sysprof-spawnable.h',
   'sysprof-symbol-dirs.h',
   'sysprof-symbol-resolver.h',
   'sysprof-symbols-source.h',