Specify CMake generator unless explicitly passed as command argument by user - cmake

Question

In all of the following, I'm using Windows, so WIN32 is always defined in CMake.
I have a CMakeLists.txt file which does its job fine, and I'm using a Preload.CMake file to specify a bunch of stuff, including a certain generator depending on circumstances:
if( WIN32 )
message( "Current generator: ${CMAKE_GENERATOR}" ) # For debugging purposes
if( NOT COMMAND cl OR NOT COMMAND nmake )
set( CMAKE_CXX_COMPILER "g++" )
set( CMAKE_GENERATOR "MinGW Makefiles" CACHE STRING "" FORCE )
set( CMAKE_MAKE_PROGRAM "make" )
message( "MSVC compiler could not be found, attempting with g++ and MinGW makefiles." )
endif( )
endif( )
This allows a Windows user with no Visual Studio to run cmake .. instead of a whole cmake .. -G "MinGW Makefiles".
It works pretty well, here is the output when cmake .. is run from an empty directory (no cache, no nothing):
Current generator:
MSVC compiler could not be found, attempting with g++ and MinGW makefiles.
Windows detected, forcing static libs.
-- The CXX compiler identification is GNU 8.2.0
-- Check for working CXX compiler: C:/MinGW/bin/g++.exe
-- Check for working CXX compiler: C:/MinGW/bin/g++.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: C:/Users/SAYENS/Desktop/picross/build
The problem with this is when I want to specify a generator through the command line. Here is what happens when I run cmake .. -G "MSYS Makefiles" from a MSYS terminal (empty directory, no cache):
Current generator:
MSVC compiler could not be found, attempting with g++ and MinGW makefiles.
CMake Error: Error: generator : MSYS Makefiles
Does not match the generator used previously: MinGW Makefiles
Either remove the CMakeCache.txt file and CMakeFiles directory or choose a different binary directory.
(There is no such file as CMakeCache.txt or a CMakeFiles directory, but explicitly specifying a generator in the command line, that is different from the one specified in Preload.CMake, seems to be enough of a source of confusion to print out this message.)
I thought I'd be able to work around the issue by "catching" the value of the -G argument with CMAKE_GENERATOR, but it turns out I can't, as per the first output line Current generator:.
How can I work this out? I tried looking for a way to parse command line arguments, but found none that doesn't involve using a wrapper script, which I'd like to avoid resorting to.

Answer 1

Set the generator name in the environment variable CMAKE_GENERATOR. The documentation says:
Specifies the CMake default generator to use when no generator is
supplied with -G. If the provided value doesn’t name a generator known
by CMake, the internal default is used. Either way the resulting
generator selection is stored in the CMAKE_GENERATOR variable.
Some generators may be additionally configured using the environment
variables:
CMAKE_GENERATOR_PLATFORM
CMAKE_GENERATOR_TOOLSET
CMAKE_GENERATOR_INSTANCE