Add pthread to LDFLAGS of Windows cross rubies

Taken from here:
  oneclick/rubyinstaller2-packages@a547657

It otherwise fails with:

16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c:8389:13: error: 'c' undeclared (first use in this function)
16 127.7  8389 |             c = CLOCK_REALTIME;
16 127.7       |             ^
16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c:8389:13: note: each undeclared identifier is reported only once for each function it appears in
16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c:8397:13: error: label 'gettime' used but not defined
16 127.7  8397 |             goto gettime;
16 127.7       |             ^~~~
16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c: In function 'rb_clock_getres':
16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c:8620:13: error: 'c' undeclared (first use in this function)
16 127.7  8620 |             c = CLOCK_REALTIME;
16 127.7       |             ^
16 127.7 /usr/local/rake-compiler/sources/ruby-3.2.9/process.c:8628:13: error: label 'getres' used but not defined
16 127.7  8628 |             goto getres;
16 127.7       |             ^~~~
16 127.7 make: *** [Makefile:468: process.o] Error 1

Author: larskanis

Dockerfile.mri.erb

@@ -167,13 +167,14 @@ else
 end
 
 strip = '-s' if platform !~ /darwin|aarch64-mingw/
+lpthread = '-lpthread' if platform =~ /mingw/
 
 xrubies_build_plan.each do |xrubies, bootstrap_ruby_version| %>
 RUN bash -c " \
   rbenv shell <%= bootstrap_ruby_version %> && \
   export CPPFLAGS='<%= "-D__USE_MINGW_ANSI_STDIO=1" if platform =~ /x64-mingw-ucrt/ %>' && \
   export CFLAGS='-O1 -fno-omit-frame-pointer -fno-fast-math -fstack-protector-strong <%= strip %>' && \
-  export LDFLAGS='-pipe <%= strip %>' && \
+  export LDFLAGS='-pipe <%= strip %> <%= lpthread %>' && \
   <%= "export LIBS='-l:libssp.a' &&" if platform =~ /mingw/ %> \
   <%= "export CC=#{target}-clang &&" if platform =~ /darwin/ %> \
   export MAKE='make V=1' && \