Allow libgit2 build to be enforced

This can be useful on machines where libgit2 is installed due to other
applications depending on it, but where the composition of this
installation does not properly work with the controller.

Reason the system version is still preferred, is because this lowers the
barrier for drive-by contributors, as a working set of (Git) dependencies
should only really be required if you are going to perform work in that
domain.

Signed-off-by: Hidde Beydals <hello@hidde.co>

Makefile

@@ -18,8 +18,10 @@ CRD_OPTIONS ?= crd:crdVersions=v1
 # Repository root based on Git metadata
 REPOSITORY_ROOT := $(shell git rev-parse --show-toplevel)
 
-# Dependency versions
+# Libgit2 version
 LIBGIT2_VERSION ?= 1.1.1
+
+# Other dependency versions
 ENVTEST_BIN_VERSION ?= 1.19.2
 KUBEBUILDER_ASSETS ?= $(shell $(SETUP_ENVTEST) use -i $(ENVTEST_BIN_VERSION) -p path)
 
@@ -31,6 +33,10 @@ LIBGIT2_PATH := $(REPOSITORY_ROOT)/hack/libgit2
 LIBGIT2_LIB_PATH := $(LIBGIT2_PATH)/lib
 LIBGIT2 := $(LIBGIT2_LIB_PATH)/libgit2.so.$(LIBGIT2_VERSION)
 
+ifneq ($(LIBGIT2_VERSION),$(SYSTEM_LIBGIT2_VERSION))
+	LIBGIT2_FORCE ?= 1
+endif
+
 # API (doc) generation utilities
 CONTROLLER_GEN_VERSION ?= v0.5.0
 GEN_API_REF_DOCS_VERSION ?= 0.3.0
@@ -164,8 +170,7 @@ endif
 libgit2: $(LIBGIT2)  ## Detect or download libgit2 library
 
 $(LIBGIT2):
-ifeq ($(LIBGIT2_VERSION),$(SYSTEM_LIBGIT2_VERSION))
-else
+ifeq (1, $(LIBGIT2_FORCE))
 	@{ \
 	set -e; \
 	mkdir -p $(LIBGIT2_PATH); \