[vcpkg-cmake] Fix usage of CMAKE_CXX_COMPILER_TARGET

[dg0yt]

• What does your PR fix?

  Fixes [boost-exception] build failure #24075 (mingw native or cross).
  Amends [boost-modular-build-helper] Attempt to pass flags more correctly into b2 #23001 - CC @ras0219-msft
  More details in [vcpkg-cmake] Fix usage of CMAKE_CXX_COMPILER_TARGET #24101 (comment).

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all, no

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  yes

[dg0yt]

For mingw gcc, we get CMAKE_CXX_COMPILER_TARGET (value x86_64-windows-gnu) but not CMAKE_CXX_COMPILE_OPTIONS_TARGET. This leads to boost exception compiling command lines and errors like

    "D:/msys64/mingw64/bin/x86_64-w64-mingw32-g++.exe"   -g "x86_64-windows-gnu" -m64 -mthreads -O0 -fno-inline -Wall -g  -DBOOST_ALL_NO_LIB=1  -I"..\include" -I"D:\msys64\home\Darer\vcpkg\installed\x64-mingw-static\include"  -c -o "D:\msys64\home\Darer\vcpkg\buildtrees\boost-exception\x64-mingw-static-dbg\boost\build\7bca12b6b2cf72a95198e22cff26ec25\clone_current_exception_non_intrusive.o" "..\src\clone_current_exception_non_intrusive.cpp"

x86_64-w64-mingw32-g++.exe: warning: x86_64-windows-gnu: linker input file unused because linking not done
x86_64-w64-mingw32-g++.exe: error: x86_64-windows-gnu: linker input file not found: No such file or directory

due the stand-alone "x86_64-windows-gnu".

Before #23001, this didn't happen because CMAKE_CXX_COMPILER_TARGET was never used without CMAKE_CXX_COMPILE_OPTIONS_TARGET:

vcpkg/ports/boost-modular-build-helper/CMakeLists.txt

Lines 143 to 151 in 93d397c

	if(CMAKE_CXX_COMPILER_TARGET AND CMAKE_CXX_COMPILE_OPTIONS_TARGET)
	if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
	string(APPEND CXXFLAGS " <compileflags>${CMAKE_CXX_COMPILE_OPTIONS_TARGET}${CMAKE_CXX_COMPILER_TARGET}")
	string(APPEND LDFLAGS " <linkflags>${CMAKE_CXX_COMPILE_OPTIONS_TARGET}${CMAKE_CXX_COMPILER_TARGET}")
	else()
	string(APPEND CXXFLAGS " <compileflags>${CMAKE_CXX_COMPILE_OPTIONS_TARGET} <compileflags>${CMAKE_CXX_COMPILER_TARGET}")
	string(APPEND LDFLAGS " <linkflags>${CMAKE_CXX_COMPILE_OPTIONS_TARGET} <linkflags>${CMAKE_CXX_COMPILER_TARGET}")
	endif()
	endif()

AFAIU Clang always takes that parameter, so I didn't restore the guard before that line.

[Masaiki]

@LilyWangLL request for a review.

[strega-nil-ms]

Thanks!