KVM: selftests: Include lib.mk before consuming $(CC)

Include lib.mk before consuming $(CC) and document that lib.mk overwrites
$(CC) unless make was invoked with -e or $(CC) was specified after make
(which makes the environment override the Makefile).  Including lib.mk
after using it for probing, e.g. for -no-pie, can lead to weirdness.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/Makefile

@@ -180,6 +180,11 @@ TEST_GEN_PROGS += $(TEST_GEN_PROGS_$(ARCH_DIR))
 TEST_GEN_PROGS_EXTENDED += $(TEST_GEN_PROGS_EXTENDED_$(ARCH_DIR))
 LIBKVM += $(LIBKVM_$(ARCH_DIR))
 
+# lib.mak defines $(OUTPUT), prepends $(OUTPUT)/ to $(TEST_GEN_PROGS), and most
+# importantly defines, i.e. overwrites, $(CC) (unless `make -e` or `make CC=`,
+# which causes the environment variable to override the makefile).
+include ../lib.mk
+
 INSTALL_HDR_PATH = $(top_srcdir)/usr
 LINUX_HDR_PATH = $(INSTALL_HDR_PATH)/include/
 LINUX_TOOL_INCLUDE = $(top_srcdir)/tools/include
@@ -205,10 +210,6 @@ pgste-option = $(call try-run, echo 'int main(void) { return 0; }' | \
 LDLIBS += -ldl
 LDFLAGS += -pthread $(no-pie-option) $(pgste-option)
 
-# After inclusion, $(OUTPUT) is defined and
-# $(TEST_GEN_PROGS) starts with $(OUTPUT)/
-include ../lib.mk
-
 LIBKVM_C := $(filter %.c,$(LIBKVM))
 LIBKVM_S := $(filter %.S,$(LIBKVM))
 LIBKVM_C_OBJ := $(patsubst %.c, $(OUTPUT)/%.o, $(LIBKVM_C))