Remove MEGAGLEST_VERSION for MG_VERSION_H_CONTENTS

I noticed that mk/macos/CMakeLists.txt has special code for extracting the
megaglest version , but its already done in CMakeLists.txt. This removes the
duplicate detection and changes the users of MEGAGLEST_VERSION to
MG_VERSION_H_CONTENTS.
Revert change to quoting

Accidentally included a change of quoting which isn't related to the diff at
hand; reverted with this commit.
Merge pull request #40 from goetzk/remove-mac-specific-version-detection-code

Remove mac specific version detection code