Make the build logic more robust for BSD systems

This properly sets MAKE (when undefined) 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+.

makefile_include.mk

@@ -13,9 +13,23 @@ 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
 
@@ -24,7 +38,12 @@ AR:=$(CROSS_COMPILE)ar
 ARFLAGS:=r
 
 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
 
 ifndef INSTALL_CMD