[CMake] Set CMAKE_INSTALL_FULL_LIBEXECDIR only for Linux

On FreeBSD, it breaks the port's packaging under Poudriere, refer to freebsd/freebsd-ports@eb6c9c8.
Related to #360

CMakeLists.txt

@@ -15,10 +15,12 @@ project(cpu-x
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
 include(GNUInstallDirs)
-if(NOT EXISTS ${CMAKE_INSTALL_FULL_LIBEXECDIR})
+# Avoid to put files under /usr/libexec or /usr/local/libexec on Linux when the path does not exist
+# On FreeBSD, /usr/local/libexec is missing under Poudriere (https://gitlab.com/FreeBSD/freebsd-ports/-/commit/eb6c9c8c4aa5981078c6b9b69527ecdaee5f22c9)
+if(${CMAKE_SYSTEM_NAME} MATCHES "Linux" AND NOT EXISTS ${CMAKE_INSTALL_FULL_LIBEXECDIR})
 	set(CMAKE_INSTALL_LIBEXECDIR      lib/${CMAKE_PROJECT_NAME})
 	set(CMAKE_INSTALL_FULL_LIBEXECDIR ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBEXECDIR})
-endif(NOT EXISTS ${CMAKE_INSTALL_FULL_LIBEXECDIR})
+endif(${CMAKE_SYSTEM_NAME} MATCHES "Linux" AND NOT EXISTS ${CMAKE_INSTALL_FULL_LIBEXECDIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/output/bin/)
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/output/lib/)
 set(CMAKE_DATA_OUTPUT_DIRECTORY    ${CMAKE_BINARY_DIR}/output/data/)