@stefano I was able to build it from the dev branch with gmake and the following patch.

diff --git a/make-bsd.mk b/make-bsd.mk
index ce0b4dde..9f077e4d 100644
--- a/make-bsd.mk
+++ b/make-bsd.mk
@@ -1,6 +1,8 @@
# This requires GNU make, which is typically "gmake" on BSD systems

INCLUDES=-isystem ext -Iext/prometheus-cpp-lite-1.0/core/include -Iext/prometheus-cpp-lite-1.0/simpleapi/include
+OTEL_INSTALL_DIR=ext/opentelemetry-cpp-api-only
+INCLUDES+=-I${OTEL_INSTALL_DIR}/include
DEFS=
LIBS=