Can't build shlib using ucrt64

[Morilli]

I'm trying to build the shlib target from source, and it fails with the following error message:

$ ./scripts/build_shlib.sh 
cd libgambatte && scons shlib 
scons: Reading SConscript files ...
Checking for C header file zlib.h... no

scons: *** Two environments with different actions were specified for the same target: src\bitmap_font.obj
(action 1: cl /Fobitmap_font.obj /c bitmap_font.cpp -Wall -Wextra -O2 -fomit-frame-pointer -fno-exceptions -fno-rtti -std=c++11 -DHAVE_STDINT_H /nologo)
(action 2: cl /Fobitmap_font.obj /c bitmap_font.cpp -Wall -Wextra -O2 -fomit-frame-pointer -fno-exceptions -fno-rtti -std=c++11 -DHAVE_STDINT_H -DSHLIB -DREVISION=830 /nologo)
File "D:\GitHub\BizHawk\submodules\gambatte\libgambatte\SConstruct", line 101, in <module>

Apparently, cl is detected and preferred over gcc, even though cl isn't on path and gcc is (thanks scons).
The documentation specifies the following: On Windows systems which identify as win32, scons searches in order for the Microsoft Visual C++ tools, the MinGW tool chain, the Intel compiler tools, and the PharLap ETS compiler. On Windows system which identify as cygwin (that is, if scons is invoked from a cygwin shell), the order changes to prefer the GCC toolchain over the MSVC tools.
Clearly, cygwin != msys so scons tries to use cl.

To force mingw(-ish) compilers to be used, the following diff can be applied:

diff --git a/libgambatte/SConstruct b/libgambatte/SConstruct
index 6204abd0..64f75734 100644
--- a/libgambatte/SConstruct
+++ b/libgambatte/SConstruct
@@ -7,6 +7,7 @@ vars.Add('CXX')

 import os
 env = Environment(ENV = os.environ,
+                  tools = ['mingw'],
                   CPPPATH = ['src', 'include', '../common'],
                   CFLAGS = global_cflags + global_defines,
                   CXXFLAGS = global_cxxflags + global_defines,

However, building with that still gives me a similar error, just with g++ instead of cl:

$ ./scripts/build_shlib.sh 
cd libgambatte && scons shlib 
scons: Reading SConscript files ...
Checking for C header file zlib.h... yes

scons: *** Two environments with different actions were specified for the same target: src\bitmap_font.o
(action 1: g++ -o bitmap_font.o -c -Wall -Wextra -O2 -fomit-frame-pointer -fno-exceptions -fno-rtti -std=c++11 -DHAVE_STDINT_H -DHAVE_ZLIB_H bitmap_font.cpp)
(action 2: g++ -o bitmap_font.o -c -Wall -Wextra -O2 -fomit-frame-pointer -fno-exceptions -fno-rtti -std=c++11 -DHAVE_STDINT_H -DSHLIB -DREVISION=830 -DHAVE_ZLIB_H bitmap_font.cpp)
File "D:\GitHub\BizHawk\submodules\gambatte\libgambatte\SConstruct", line 102, in <module>