Default the compiler only reuses the precompiled units for speed. Mixing the precompiled units and the source is not recommended, specially for beginners.
So that practically means rebuilding the FPC distribution, and update(**) the binary files in $prefix/lib/fpc/$FPCVERSION/units/$FPCTARGET/* and then recompile lazarus.
When done properly, on linux FPC rebuilds in about a fat minute, maybe one and an half minute on a average quadcore or more machine, even if a bit older (like a i5-2500)
(**)Best is actually to remove/move away the old files, and put the new ones instead.