Add /etc/fstab in order to remove /cygdrive/ prefix (#17138)

* De-duplicate MSYS2 PATH_TO_ROOT construction

* Create MSYS2 /etc/fstab

scripts/cmake/vcpkg_acquire_msys.cmake

@@ -531,6 +531,12 @@ function(vcpkg_acquire_msys PATH_TO_ROOT_OUT)
         endforeach()
         file(RENAME ${PATH_TO_ROOT}.tmp ${PATH_TO_ROOT})
     endif()
-    message(STATUS "Using msys root at ${DOWNLOADS}/tools/msys2/${TOTAL_HASH}")
+    # Due to skipping the regular MSYS2 installer,
+    # some config files need to be established explicitly.
+    if(NOT EXISTS "${PATH_TO_ROOT}/etc/fstab")
+        # This fstab entry removes the cygdrive prefix from paths.
+        file(WRITE "${PATH_TO_ROOT}/etc/fstab" "none  /  cygdrive  binary,posix=0,noacl,user  0  0")
+    endif()
+    message(STATUS "Using msys root at ${PATH_TO_ROOT}")
     set(${PATH_TO_ROOT_OUT} ${PATH_TO_ROOT} PARENT_SCOPE)
 endfunction()

scripts/cmake/vcpkg_build_make.cmake

@@ -97,8 +97,7 @@ function(vcpkg_build_make)
         set(NO_PARALLEL_MAKE_OPTS ${_bc_MAKE_OPTIONS} -j 1 --trace -f ${_bc_MAKEFILE} ${_bc_BUILD_TARGET})
 
         string(REPLACE " " "\\\ " _VCPKG_PACKAGE_PREFIX ${CURRENT_PACKAGES_DIR})
-        # Don't know why '/cygdrive' is suddenly a requirement here. (at least for x264)
-        string(REGEX REPLACE "([a-zA-Z]):/" "/cygdrive/\\1/" _VCPKG_PACKAGE_PREFIX "${_VCPKG_PACKAGE_PREFIX}")
+        string(REGEX REPLACE "([a-zA-Z]):/" "/\\1/" _VCPKG_PACKAGE_PREFIX "${_VCPKG_PACKAGE_PREFIX}")
         set(INSTALL_OPTS -j ${VCPKG_CONCURRENCY} --trace -f ${_bc_MAKEFILE} ${_bc_INSTALL_TARGET} DESTDIR=${_VCPKG_PACKAGE_PREFIX})
         #TODO: optimize for install-data (release) and install-exec (release/debug)
     else()