cmake: add pkg-config file

pkg-config allows using the library in build systems that are not cmake,
by exporting the same information from the cmake -config files in a
buildsystem-neutral format.

Fixes #16

mimalloc.pc.in

@@ -0,0 +1,11 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+libdir=@libdir_for_pc_file@
+includedir=@includedir_for_pc_file@
+
+Name: @PROJECT_NAME@
+Description: a compact general purpose allocator with excellent performance
+Version: @PROJECT_VERSION@
+URL: https://github.com/microsoft/mimalloc/
+Libs: -L${libdir} -lmimalloc
+Libs.private: @pc_libraries@
+Cflags: -I${includedir}