CMake: properly set default build type

CMakeLists.txt

@@ -8,7 +8,9 @@ option(DISABLE_STATIC "Avoid building/installing static libraries.")
 option(LONG_OUTPUT_NAMES "Use longer names for binaries and libraries: squirrel3 (not sq).")
 
 set(CMAKE_INSTALL_PREFIX "${CMAKE_SOURCE_DIR}" CACHE PATH "")
-set(CMAKE_BUILD_TYPE "Release" CACHE STRING "")
+if (NOT CMAKE_BUILD_TYPE)
+  set(CMAKE_BUILD_TYPE "Release")
+endif ()
 
 project(squirrel C CXX)