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

diff --git a/tools/FindPythonLibsNew.cmake b/tools/FindPythonLibsNew.cmake
index c9b95a9..9ea6036 100644
--- a/tools/FindPythonLibsNew.cmake
+++ b/tools/FindPythonLibsNew.cmake
@@ -144,7 +144,7 @@
 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")