cmake: Allow user to override CMAKE_DEBUG_POSTFIX

[webmaster128]

In #1649 the cache variable CMAKE_DEBUG_POSTFIX was set to "-d" by FORCE, giving the user no chance to override it to the desired value. This change keeps "-d" as default but allows users to change the value.

Current master

$ mkdir build && cd build
$ cmake -DCMAKE_BUILD_TYPE=Debug ..
$ DESTDIR=installation make install

-- Installing: installation/usr/local/lib/libcurl-d.so

$ mkdir build && cd build
$ cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_DEBUG_POSTFIX=_bar ..
$ DESTDIR=installation make install

-- Installing: installation/usr/local/lib/libcurl-d.so

This PR

$ mkdir build && cd build
$ cmake -DCMAKE_BUILD_TYPE=Debug ..
$ DESTDIR=installation make install

-- Installing: installation/usr/local/lib/libcurl-d.so

$ mkdir build && cd build
$ cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_DEBUG_POSTFIX=_bar ..
$ DESTDIR=installation make install

-- Installing: installation/usr/local/lib/libcurl_bar.so

cc @paulharris

[mention-bot]

@webmaster128, thanks for your PR! By analyzing the history of the files in this pull request, we identified @snikulov, @billhoffman and @Sukender to be potential reviewers.

[coveralls]

Coverage increased (+0.02%) to 75.099% when pulling 0b2931a on webmaster128:custom_suffix into 77cd4e7 on curl:master.

[bagder]

Seems totally reasonable to me!

[bagder]

Thanks!