What I did was only bump the version in the pclos specfile to version 120. The build works without any errors and creates the three rpms of x264, devel, and static.
I *think* the issue is that ldconfig shows:
libx264.so.115 --> libx264.so.115
even though there is an installed link from libx264.so --> libx264.so.115.
My thinking is that a shared library should have a link of "libsomelib.so", "libsomelib.so.0", or both(?) to the real library,
so that programs link to the link and not the real library, making library upgrades possible without breaking things.
Since ldconfig shows a "hardwired" libx264.so.115, and not to a ".so", any rpm installed program that needs x264 is going to use the hardwired lib. On any attempt to upgrade x264, rpm will then show errors with those programs because they "need" that hardwired version. ( Did that make sense?
For a test I recompiled the pclos 115 src rpm and reinstalled, and ldconfig still did not show the ".so" link, even though it is installed.
I am no expert on libs, shared libs, and ldconfig, and this is all a best guess!