cmake: only install PDB files with msvc

Otherwise cmake windows builds with the mingw compiler are broken.

build/cmake/CMakeLists.txt

@@ -194,7 +194,7 @@ install(FILES ${GLEW_DIR}/glew.pc
         DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
 )
 
-if(WIN32 AND (NOT MSVC_VERSION LESS 1600) AND (NOT CMAKE_VERSION VERSION_LESS "3.1"))
+if(WIN32 AND MSVC AND (NOT MSVC_VERSION LESS 1600) AND (NOT CMAKE_VERSION VERSION_LESS "3.1"))
     install(
         FILES $<TARGET_PDB_FILE:glew>
         DESTINATION ${CMAKE_INSTALL_LIBDIR}