Disable AMReX_FLATTEN_FOR by default (#3860)

Flattening (#3855) makes compilation too slow for some codes. For
example, on my desktop, the compilation time for Castro_react.cpp
increased from 3 minutes to 49 minutes.

Author: WeiqunZhang

Docs/sphinx_documentation/source/BuildingAMReX.rst

@@ -532,8 +532,8 @@ The list of available options is reported in the :ref:`table <tab:cmakevar>` bel
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
    | AMReX_PROBINIT               |  Enable support for probin file                 | Platform dependent      | YES, NO               |
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
-   | AMReX_FLATTEN_FOR            |  Enable flattening of ParallelFor and similar   | YES unless for Debug    | YES, NO               |
-   |                              |  functions for host code                        | build                   |                       |
+   | AMReX_FLATTEN_FOR            |  Enable flattening of ParallelFor and similar   | NO                      | YES, NO               |
+   |                              |  functions for host code                        |                         |                       |
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
 .. raw:: latex
 

Tools/CMake/AMReXOptions.cmake

@@ -377,11 +377,7 @@ endif ()
 
 print_option( AMReX_ASSERTIONS )
 
-if ( "${CMAKE_BUILD_TYPE}" MATCHES "Debug" )
-   option( AMReX_FLATTEN_FOR "Enable flattening of ParallelFor and other similar functions" OFF)
-else ()
-   option( AMReX_FLATTEN_FOR "Enable flattening of ParallelFor and other similar functions" ON)
-endif ()
+option( AMReX_FLATTEN_FOR "Enable flattening of ParallelFor and other similar functions" OFF)
 print_option( AMReX_FLATTEN_FOR )
 
 option(AMReX_BOUND_CHECK  "Enable bound checking in Array4 class" OFF)

Tools/GNUMake/Make.defs

@@ -314,7 +314,7 @@ endif
 ifdef USE_FLATTEN_FOR
   USE_FLATTEN_FOR := $(strip $(USE_FLATTEN_FOR))
 else
-  USE_FLATTEN_FOR := TRUE
+  USE_FLATTEN_FOR := FALSE
 endif
 
 ifdef WARN_ALL