build(Makefile): avoid building if LDFLAGS returns error checking for SemVer

[egypcio]

What this PR does / why we need it:

hack/version.sh doesn't properly block the build if a SemVer is not found in git tags; it returns an erros via exit 1 but still passes its value to go build and it tries to move further with the build process - unnecessarily.

by merging this change, we:

• fix properly halting build if SemVer is not found directly in the Makefile;
• check if LDFLAGS was able to identify a Semantic Version tag in the repository;
• avoid passing broken string values to go build.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

• before patching

$ git tag -s v99.rs.belgrade
$ make clusterctl
go build -trimpath -ldflags "GIT_VERSION should be a valid Semantic Version. Current value: v99.rs.belgrade-dirty Please see more details here: https://semver.org" -o bin/clusterctl sigs.k8s.io/cluster-api/cmd/clusterctl
invalid value "GIT_VERSION should be a valid Semantic Version. Current value: v99.rs.belgrade-dirty Please see more details here: https://semver.org" for flag -ldflags: missing =<value> in <pattern>=<value>
usage: go build [-o output] [build flags] [packages]
Run 'go help build' for details.
make: *** [clusterctl] Error 2

• after patching

$ git tag -s v99.br.rio-de-janeiro
$ make clusterctl
Makefile:268: SEMVER_CHECK: GIT_VERSION should be a valid Semantic Version. Current value: v99.br.rio-de-janeiro Please see more details here: https://semver.org
Makefile:269: *** Build halted.  Stop.

$ git tag -s v99.123.0
$ make clusterctl
go build -trimpath -ldflags "-X 'sigs.k8s.io/cluster-api/version.buildDate=2026-03-08T08:30:39Z' -X 'sigs.k8s.io/cluster-api/version.gitCommit=82699d020ac47d94061afd0bc62dfe7f0c23a858' -X 'sigs.k8s.io/cluster-api/version.gitTreeState=clean' -X 'sigs.k8s.io/cluster-api/version.gitMajor=99' -X 'sigs.k8s.io/cluster-api/version.gitMinor=123' -X 'sigs.k8s.io/cluster-api/version.gitVersion=v99.123.0' -X 'sigs.k8s.io/cluster-api/version.gitReleaseCommit=82699d020ac47d94061afd0bc62dfe7f0c23a858'" -o bin/clusterctl sigs.k8s.io/cluster-api/cmd/clusterctl

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @egypcio. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[egypcio]

/area clusterctl

[egypcio]

/area misc

[egypcio]

rebased ✅

[egypcio]

ping? 🤓

• @chrischdi
• @Karthik-K-N