lib/googletest: also use -O0 for internal tests with GCC

This fixes the build with gcc 16's aggressive inlining.

Reviewed by:	ngie
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D59538

lib/googletest/tests/Makefile.inc

@@ -5,14 +5,14 @@ TESTSDIR=	${TESTSBASE}/lib/googletest/${.CURDIR:T}
 
 PACKAGE=	tests
 
-# Clang's optimizer spends a really long time on these tests at -O2. Changing
+# Both GCC's and Clang's optimizer spends a really long time on these tests at -O2. Changing
 # -O2 to -O1 reduces the -j32 time for lib/googletest/test from 131s to 71s.
 # Using -O0 further reduces the time to 29s, and also reduces the disk usage
 # from 144MB (at -O2) / 92MB (at -O1) to 82MB, so we use -O0.
 # Note: Building without debug info saves about 10-15% of the build time, so we
 # only enable debug info if DEBUG_FLAGS is not empty (71s -> 64s at -O1 and -j32).
-CFLAGS.clang+=	-O0
+CFLAGS+=	-O0
 .if empty(DEBUG_FLAGS)
 MK_DEBUG_FILES:=no
-CFLAGS.clang+=	-g0
+CFLAGS+=	-g0
 .endif