cmake: Fix OBS_INSTALL_PREFIX

Before 349372b3b3, OBS_INSTALL_PREFIX contained trailing slash so that
it can concatinate with further paths.

UI/cmake/os-freebsd.cmake

@@ -1,5 +1,5 @@
 target_sources(obs-studio PRIVATE platform-x11.cpp)
-target_compile_definitions(obs-studio PRIVATE OBS_INSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}")
+target_compile_definitions(obs-studio PRIVATE OBS_INSTALL_PREFIX="${OBS_INSTALL_PREFIX}")
 target_link_libraries(obs-studio PRIVATE Qt::GuiPrivate procstat)
 
 if(TARGET OBS::python)

UI/cmake/os-linux.cmake

@@ -1,5 +1,5 @@
 target_sources(obs-studio PRIVATE platform-x11.cpp)
-target_compile_definitions(obs-studio PRIVATE OBS_INSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}")
+target_compile_definitions(obs-studio PRIVATE OBS_INSTALL_PREFIX="${OBS_INSTALL_PREFIX}")
 target_link_libraries(obs-studio PRIVATE Qt::GuiPrivate)
 
 if(TARGET OBS::python)

libobs/cmake/legacy.cmake

@@ -448,7 +448,7 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/obsconfig.h.in ${CMAKE_BINARY_DIR}/co
 target_compile_definitions(
   libobs
   PUBLIC HAVE_OBSCONFIG_H
-  PRIVATE "OBS_INSTALL_PREFIX=\"${CMAKE_INSTALL_PREFIX}\"" "OBS_QT_VERSION=${_QT_VERSION}"
+  PRIVATE "OBS_INSTALL_PREFIX=\"${OBS_INSTALL_PREFIX}\"" "OBS_QT_VERSION=${_QT_VERSION}"
           "$<$<BOOL:${LINUX_PORTABLE}>:LINUX_PORTABLE>")
 
 if(ENABLE_FFMPEG_MUX_DEBUG)