FROMLIST: Makefile: fix conditional around sparse

sparse would always be invoked, since `ifdef C` is always true afet `C ?= xx`.
Instead, use ifeq to check if C is 0 or 1.

Fixes f884bfe684f ("mmc-utils: Make functions static for local scope enforcement")
(am from https://patchwork.kernel.org/patch/13956017/)
(also found at https://lore.kernel.org/r/20250201012936.2816101-1-gwendal@chromium.org)

BUG=b:393646176
TEST=Check with emake, emake C=1, emake C=0 that sparse not used in the
later invocation.

Change-Id: I29488386a4b1e5a32de4e174eb1289d4a8671fc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/mmc-utils/+/6222513
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Commit-Queue: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
diff --git a/Makefile b/Makefile
index 06ae0f7..c0284bb 100644
--- a/Makefile
+++ b/Makefile
@@ -27,14 +27,14 @@
 
 # make C=1 to enable sparse - default
 C ?= 1
-ifdef C
+ifeq "$(C)" "1"
 	check = sparse $(CHECKFLAGS) $(AM_CFLAGS)
 endif
 
 all: $(progs)
 
 .c.o:
-ifdef C
+ifeq "$(C)" "1"
 	$(check) $<
 endif
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(DEPFLAGS) -c $< -o $@