This is a very specific bug I encountered when trying to build example programs of a library using Scons under MSYS2 (64 bit version). Although this is actually a version of SDL 2.0.12 with some patches from 2.0.13 which was rebuilt by the MSYS2 team, I think this bug should actually be fixed here. 

Version 2.0.12-4 worked fine, but the bug started to appear in 2.0.12-5. The only difference between both is the application of a patch based on Hg commit id fc03d19926ed in the SDL repository. 

The error message I get now is:
g++ -o examples/sdl2widgets.exe -Wl,-no-undefined -mwindows -Wl,--no-undefined examples/sdl2widgets.o -Lsrc -L/mingw64/lib -lguisan -lSDL2_image -lSDL2_ttf -lSDL2main -lSDL2 -lmingw32 -ldxerr8 -ldinput8 -lshell32 -lsetupapi -ladvapi32 -luuid -lversion -loleaut32 -lole32 -limm32 -lwinmm -lgdi32 -luser32 -lm
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.2.0/../../../../x86_64-w64-mingw32/lib/../lib/libmingw32.a(lib64_libmingw32_a-crt0_c.o): in function `main':
D:/mingwbuild/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/crt0_c.c:18: undefined reference to `WinMain'
collect2.exe: error: ld returned 1 exit status
scons: *** [examples/sdl2widgets.exe] Error 1
scons: building terminated because of errors.

After some digging, scons takes its library list in this case from a call to pkg-config, which itself relies on the contents of the generated sdl2.pc file. It turns out that in this file, and despite applying the patch fc03d19926ed, a library still appears both in the Libs and in the Libs.private section: mingw32. Consequently, the call to pkg-config --cflags --libs sdl2 contains both the Libs and the Libs.private section in this order, but the subsequent use of the ParseConfig method in scons only leaves the rightmost instance of -lmingw32 in the compilation line. Editing manually the compilation command to put -lmingw32 BEFORE -lSDL2main solves the error. 

My guess here is that to fix this issue, line 1577 of the CMakeLists.txt file (which reads  list(APPEND EXTRA_LIBS mingw32)) should be removed.