Use CMAKE_CURRENT_*_DIR when finding gcem #1282

Job	Run time
🔧 MSVC Additional (default)	2m 6s
🔨 MSVC x86 glib	2m 24s
🔧 MinGW fluidsynth-mingw-static x64	3m 30s
🔨 MSVC x64 glib	2m 16s
🔨 MSVC x86 cpp11	2m 20s
🔧 MSVC Additional (no_network)	2m 2s
🔨 MSVC x64 cpp11	2m 25s
🔧 MSVC Additional (static_lib)	2m 10s
🔧 MSVC Additional (debug_prof)	2m 9s
🔧 MSVC Additional (minimal)	1m 54s
🚧🟧 clang64 cpp11	4m 14s
🚧🟨 ucrt64 cpp11	3m 43s
🚧🟧 clang64 glib	3m 49s
🔧 MSVC Additional (osal_cpp11)	2m 21s
🚧⬛ mingw32 glib	3m 18s
🚧🟦 mingw64 glib	3m 52s
🚧⬛ mingw32 cpp11	3m 59s
🚧🟦 mingw64 cpp11	3m 42s
🔧 MinGW fluidsynth-mingw x64	2m 2s
🚧🟨 ucrt64 glib	3m 28s
🧪 Acceptance Test	5s
🚀 Publish Release Artifacts	-1s
📢 Announce Release	0s
	57m 48s

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