cmake: Do not hardcode CMAKE_INSTALL_PREFIX

Author: nfnty

CMakeLists.txt

@@ -19,7 +19,9 @@ find_package(Go 1.4 REQUIRED)
 find_package(Git REQUIRED)
 find_package(Protobuf 2.3 QUIET)
 set(CPACK_PACKAGE_FILE_NAME ${CMAKE_PROJECT_NAME}-${CPACK_PACKAGE_VERSION_MAJOR}_${CPACK_PACKAGE_VERSION_MINOR}_${CPACK_PACKAGE_VERSION_PATCH}-${GO_PLATFORM}-${GO_ARCH})
-set(CMAKE_INSTALL_PREFIX ${CMAKE_PROJECT_NAME})
+if (NOT DEFINED CMAKE_INSTALL_PREFIX)
+    set(CMAKE_INSTALL_PREFIX ${CMAKE_PROJECT_NAME})
+endif()
 set(PROJECT_PATH "${CMAKE_BINARY_DIR}/heka")
 set(HEKA_PATH "${PROJECT_PATH}/src/github.com/mozilla-services/heka")
 set(LUA_INCLUDE_PATH "${PROJECT_PATH}/include")