Changes to compile on macOS and some minor additions

[aramallo]

macOS support

Makefile changes

• Main changes are to fix compilation and linking issues found due to the way macOS manages dynamic libs, this includes the use of install_name_tool to change the libcozo_c.so library name and path, so that the linker can find it. The dlopen() call to fail as macOS tries to find the lib in standard locations ignoring env var flags and the ld rpath option.
• Added CC_FLAGS required on macOS for compilation to succeed (-O3 -std=c99 -finline-functions -Wall -Wmissing-prototypes)
• ASDF as optional - the makefile now computes the ERTS library locations using ASDF if present only, otherwise just asumes there is an erl executable in the users path and callserlang -eval` to determine the different library locations (this works for KERL or manually installed Erlang runtimes) find it

cozo_nif changes

• added fallback support for code:priv_dir(cozy) which might be required when embedding the lib on releases

[niamtokik]

Looks good, only one problem in Makefile (I will fix it myself after the PR).

diff --git a/Makefile b/Makefile
index 20cc9d3..f17059f 100644
--- a/Makefile
+++ b/Makefile
@@ -23,6 +23,7 @@ ifdef ASDF_DIR
        ASDF_ERLANG_VERSION ?= $(shell asdf current erlang | awk '{print $$2}')
        ERTS_INCLUDE_DIR ?= $(ASDF_DIR)/installs/erlang/$(ASDF_ERLANG_VERSION)/usr/include
        ERL_INTERFACE_LIB_DIR ?= $(ASDF_DIR)/installs/erlang/$(ASDF_ERLANG_VERSION)/usr/lib
+       ERL_INTERFACE_INCLUDE_DIR ?= $(ASDF_DIR)/installs/erlang/$(ASDF_ERLANG_VERSION)/usr/include
 else
        ERL_BIN_DIR ?= $(shell erl -noshell -eval "io:format(\"~ts/erts-~ts/bin/\", [code:root_dir(), erlang:system_info(version)])." -s init stop)
        ERTS_INCLUDE_DIR ?= $(shell erl -noshell -eval "io:format(\"~ts/erts-~ts/include/\", [code:root_dir(), erlang:system_info(version)])." -s init stop)
@@ -85,7 +86,7 @@ COZO_HEADER_URL = $(COZO_REPOSITORY)/v$(COZO_VERSION)/$(COZO_HEADER_NAME)
 CC_FLAGS ?= -O3 -std=c99 -finline-functions -Wall -Wmissing-prototypes -I $(ERTS_INCLUDE_DIR) -L $(ERL_INTERFACE_LIB_DIR)
 CC_INCLUDE = -I $(ERL_INTERFACE_INCLUDE_DIR) -I $(LOCAL_DIR)/c_src -I $(LOCAL_DIR)/$(PRIV_DIR)
 LDLIBS = -L $(LOCAL_DIR)/c_src -L $(LOCAL_DIR)/$(PRIV_DIR)
-CC_OPTS ?= $(CC_INCLUDE) $(LDLIBS) -lei -lcozo_c -fPIC $(CC_FLAGS) $(LDFLAGS)
+CC_OPTS ?= $(CC_INCLUDE) $(LDLIBS) -Werror -lei -lcozo_c -fPIC $(CC_FLAGS) $(LDFLAGS)