load fmt as external fail

[westfly]

the tag of fmt has no v prefix

6.1.0
6.1.1
6.1.2
qls
v0.11.0

default strategy will add v prefix to VERSION supplying by user.

148   if (NOT DEFINED CPM_ARGS_GIT_TAG) 
149     set(CPM_ARGS_GIT_TAG v${CPM_ARGS_VERSION}) 
150   endif()

should we give an option to handle this situation as well as VERSION equal master?

[TheLartians]

Packages not using v-prefixed tags require specifying GIT_TAG explicitly and additionally to VERSION, which I agree isn't optimal. I think it makes sense to assume the v-prefix by default as it seems to be most common convention on GitHub, but also to provide an option to override this on a per-package basis.

Maybe we should add an option VERSION_PREFIX to the package definition to avoid the duplicate version specification. e.g.

CPMAddPackage(
  NAME fmt
  VERSION 6.1.2
  GITHUB_REPOSITORY fmtlib/fmt
  VERSION_PREFIX ""
)

[TheLartians]

As it turns out, CMake did not support empty arguments until very recently, which is why using VERSION_PREFIX "" will not work. Currently however CPM can take the version from GIT_TAG as well, reducing the need for double specification as well. So the current "correct" way to add fmt would be the following.

CPMAddPackage(
  NAME fmt
  GIT_TAG 6.1.2 # CPM can detect the version from this tag
  GITHUB_REPOSITORY fmtlib/fmt
)

I will update the documentation and add fmt as an example accordingly.