build: don't use vcpkg on non-WIN32 w/o triplet

Fix the logic in the vcpkg automatic support to return on non-WIN32 if
VCPKG_TARGET_TRIPLET is not defined.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

cmake/Set-Toolchain-vcpkg.cmake

@@ -6,7 +6,11 @@ if(POLICY CMP0135)
     cmake_policy(SET CMP0135 NEW) # Use timestamps from archives.
 endif()
 
-if(NOT DEFINED VCPKG_TARGET_TRIPLET AND WIN32)
+if(NOT DEFINED VCPKG_TARGET_TRIPLET)
+    if(NOT WIN32)
+        return()
+    endif()
+
     # Check if we are in an MSVC environment.
     find_program(cl_exe_path NAME cl.exe HINTS ENV PATH)