This is a case of "everything used to just work" in 3.6 and below, our Gardner/Costas block relies on this functionality; i.e., it did a #include of gri_mmse_fir_interpolator_cc.h and contructed a new one via "new gri_mmse_fir_interpolator_cc()".
Of course I changed the above (respectively) for 3.7 to
... new gr::filter::mmse_fir_interpolator_cc()
However in 3.7 there is a linker failure
libgnuradio-op25_repeater.so: undefined reference to
A very quick initial diagnosis is that it used to be in libgnuradio-core.so in 3.6 and below, but now it's been split out into libgnuradio-filter.so in 3.7 - whereas the framework generated by gr_modtool only links to libgnuradio-runtime (in 3.7) which of course doesn't contain the mmse interpolator ....
As a workaround I've added a find_library() for libgnuradio-filter.so to the lib/CMakeLists.txt which seems to make everything happy... Not sure this is even a bug, just mentioning it in case someone else porting to 3.7 may be bitten by it...