I would like to instruct Inno Setup to install a certain external file only if does exist.
Like so:
Source: \"d:\\sources\\SomeDLL.dll\"; DestDir: {a
Try this flag:
onlyifdestfileexists
Only install the file if a file of the same name already exists on the user's system. This flag may be useful if your installation is a patch to an existing installation, and you don't want files to be installed that the user didn't already have.
http://www.jrsoftware.org/ishelp/index.php?topic=filessection