Linker-supplied __ImageBase must be extern "C". (#10454)
https://jenkins.mono-project.com/job/w/15083/parsed_console/log.html
assembly.obj : error LNK2001: unresolved external symbol "struct _IMAGE_DOS_HEADER __ImageBase" (?__ImageBase@@3U_IMAGE_DOS_HEADER@@A) [d:\j\workspace\w\msvc\libmono-dynamic.vcxproj]
d:\j\workspace\w\msvc/./build/boehm/Win32\bin\ReleaseCxx\mono-2.0-boehm.dll : fatal error LNK1120: 1 unresolved externals [d:\j\workspace\w\msvc\libmono-dynamic.vcxproj]
Since it is data, extern "C" w/o braces is confusing, but maybe valid, so begin/end form is used.