Add fix for cmake control of CMAKE_BUILD_TYPE only if mbedtls is the root project

Signed-off-by: Peter Toft <peter.toft@dirac.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5af4c81..27590ef 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -112,9 +112,12 @@
     endif()
 endif()
 
-set(CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE}
-    CACHE STRING "Choose the type of build: None Debug Release Coverage ASan ASanDbg MemSan MemSanDbg Check CheckFull"
-    FORCE)
+# If this is the root project add longer list of available CMAKE_BUILD_TYPE values
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+    set(CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE}
+        CACHE STRING "Choose the type of build: None Debug Release Coverage ASan ASanDbg MemSan MemSanDbg Check CheckFull"
+        FORCE)
+endif()
 
 # Create a symbolic link from ${base_name} in the binary directory
 # to the corresponding path in the source directory.