Fix the use of MSVC in an MSYS environment (pybind#2087)

Author: Baljak

tools/FindPythonLibsNew.cmake

@@ -144,7 +144,7 @@ string(REGEX REPLACE "\\\\" "/" PYTHON_PREFIX "${PYTHON_PREFIX}")
 string(REGEX REPLACE "\\\\" "/" PYTHON_INCLUDE_DIR "${PYTHON_INCLUDE_DIR}")
 string(REGEX REPLACE "\\\\" "/" PYTHON_SITE_PACKAGES "${PYTHON_SITE_PACKAGES}")
 
-if(CMAKE_HOST_WIN32 AND NOT DEFINED ENV{MSYSTEM})
+if(CMAKE_HOST_WIN32 AND NOT (MINGW AND DEFINED ENV{MSYSTEM}))
     set(PYTHON_LIBRARY
         "${PYTHON_PREFIX}/libs/Python${PYTHON_LIBRARY_SUFFIX}.lib")