depends: For mingw cross compile use -gcc-posix to prevent library conflict

CMake parses some paths from the spec of the C compiler, assuming it
will be the linker, resulting in the link to end up with
`-L/usr/lib/gcc/x86_64-w64-mingw32/12-win32` on debian bookworm if both
-win32 and -posix variants are installed, and -win32 is the default
alternative.

This results in the wrong C++ library being linked, missing
std::threads::hardware_concurrency and other threading functions.

To fix this, use the -posix variant of gcc as well when available. This
fixes a regression compared to autotools, where this scenario worked.

Github-Pull: bitcoin#31013
Rebased-From: ae56b32

Author: laanwj

depends/hosts/mingw32.mk

@@ -1,3 +1,6 @@
+ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-gcc-posix"),)
+mingw32_CC := $(host)-gcc-posix
+endif
 ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-g++-posix"),)
 mingw32_CXX := $(host)-g++-posix
 endif