Qt 5.12.0 -> 5.12.1 regression: config.tests fails out-of-system libraries

Configuring Qt for the raspberry pi in 5.12.0 used to successfully compile:

config.tests/egl-brcm

and enable the associated functionality. This config test is not even touched in Qt 5.12.1

Turns out:

        "bcm_host": {                                                                                                         
            "export": "",                                                                                                     
            "sources": [                                                                                                      
                "-lbcm_host"                                                                                                  
            ]                                                                                                                 
        }, 

now barfs with:

Trying source 0 (type inline) of library bcm_host ...                                                                         
None of [libbcm_host.so libbcm_host.a] found in [] and global paths.                                                          
 => source produced no result.          

Which makes sense as bcm_host lives in a discrete noncentral path, and we used to allow mkspecs to provide arbitrary link magic.

This is clearly not isolated to the egl-brcm config.test and must be giving anyone who tracks Qt patch releases (something I used to advocate) a really bad day.