Make the build logic more robust for BSD systems

This properly sets MAKE on BSDs to gmake rather than make, which refers
to the incompatible BSD Make. Further, it betters detection of Clang as
the default compiler, which is the case on FreeBSD 11.0+ and OpenBSD
6.0+, and it fixes the unrecognized call to arch on FreeBSD.

makefile_include.mk

@@ -17,15 +17,34 @@ ifndef CROSS_COMPILE
   CROSS_COMPILE=
 endif
 
-ifeq ($(CC),cc)
-  CC = $(CROSS_COMPILE)gcc
+# We only need to go through this dance of determining the right compiler if we're using
+# cross compilation, otherwise $(CC) is fine as-is.
+ifneq (,$(CROSS_COMPILE))
+ifeq ($(origin CC),default)
+CSTR := "\#ifdef __clang__\nCLANG\n\#endif\n"
+ifeq ($(PLATFORM),FreeBSD)
+  # XXX: FreeBSD needs extra escaping for some reason
+  CSTR := $$$(CSTR)
 endif
+ifneq (,$(shell echo $(CSTR) | $(CC) -E - | grep CLANG))
+  CC := $(CROSS_COMPILE)clang
+else
+  CC := $(CROSS_COMPILE)gcc
+endif # Clang
+endif # cc is Make's default
+endif # CROSS_COMPILE non-empty
+
 LD=$(CROSS_COMPILE)ld
 AR=$(CROSS_COMPILE)ar
 RANLIB=$(CROSS_COMPILE)ranlib
 
 ifndef MAKE
-   MAKE=make
+# BSDs refer to GNU Make as gmake
+ifneq (,$(findstring $(PLATFORM),FreeBSD OpenBSD DragonFly NetBSD))
+  MAKE=gmake
+else
+  MAKE=make
+endif
 endif
 
 CFLAGS += -I./ -Wall -Wsign-compare -Wextra -Wshadow
@@ -67,8 +86,14 @@ ifeq ($(PLATFORM), Darwin)
 CFLAGS += -Wno-nullability-completeness
 endif
 
+ifeq ($(PLATFORM),FreeBSD)
+  _ARCH := $(shell sysctl -b hw.machine_arch)
+else
+  _ARCH := $(shell arch)
+endif
+
 # adjust coverage set
-ifneq ($(filter $(shell arch), i386 i686 x86_64 amd64 ia64),)
+ifneq ($(filter $(_ARCH), i386 i686 x86_64 amd64 ia64),)
    COVERAGE = test_standalone timing
    COVERAGE_APP = ./test && ./timing
 else