Corrects Windows lib lookup in case-sensitive OSes

[luislavena]

Downcase name of Windows import libraries to avoid duplicates references (Eg. kernel32 and Kernel32) but to correctly find the libraries on case-sensitive filesystems, like when cross-compiling from Linux.

While on Windows Kernel32 and kernel32 result in the correct library found for linking, it does not work the same way when cross-compiling.

This also applies to Winsock2 (ws2_32) usage.

The change normalizes on the usage of downcased names for these libraries (which will have no impact when compiling natively on Windows).

Prior to this change:

$ crystal build --cross-compile --target x86_64-windows-gnu 1.cr
cc 1.obj -o 1  -Wl,--stack,0x800000 -L/usr/local/bin/../lib/crystal -lgc -lpthread \
  -ldl -municode -lntdll -liconv -ladvapi32 -lshell32 \
  -lole32 -lWS2_32 -lkernel32 -lKernel32

After:

$ crystal build --cross-compile --target x86_64-windows-gnu 1.cr
cc 1.obj -o 1  -Wl,--stack,0x800000 -L/usr/local/bin/../lib/crystal -lgc -lpthread \
  -ldl -municode -lntdll -liconv -ladvapi32 -lshell32 \
  -lole32 -lws2_32 -lkernel32