Make install does not copy osdp_export.h when using make build

[bricktsre]

Describe the bug
I am cross compiling libosdp for an embedded system and have to use the make build system. The steps I am following are

1. ./configure.sh --prefix .../libosdp/built --cross-compile ....
2. make
3. make install

Expected behavior
I expect make install to fully install the application including headers to the provided prefix location.

Observed behavior
osdp_export.h does not get copied to the install location. This is because configure.sh writes osdp_export.h to ${BUILD_DIR}/include/osdp_export.h but the makefile tries to copy it from $(O)/osdp_export.h with $O being defined as O ?= $(BUILD_DIR).