toolchain/toolchain-wrapper: let recent GCC handle SOURCE_DATE_EPOCH

When using precompiled headers, changing any macros defined on the
command line will invalidate the precompiled header.  With
toolchain-wrapper adding __DATE__ and __TIME__, any commits to Buildroot
will invalidate incremental builds regardless of whether the precompiled
header actually uses those values (affecting _OVERRIDE_SRCDIR).

GCC-7 and later support SOURCE_DATE_EPOCH and use it to define __DATE__
and __TIME__ internally, avoiding any impact on precompiled headers.

Disable the custom handling in toolchain-wrapper if GCC is version 7 or
newer.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

toolchain/toolchain-wrapper.c

@@ -179,6 +179,7 @@ static void check_unsafe_path(const char *arg,
 	}
 }
 
+#ifdef BR_NEED_SOURCE_DATE_EPOCH
 /* Returns false if SOURCE_DATE_EPOCH was not defined in the environment.
  *
  * Returns true if SOURCE_DATE_EPOCH is in the environment and represent
@@ -232,6 +233,15 @@ bool parse_source_date_epoch_from_env(void)
 	}
 	return true;
 }
+#else
+bool parse_source_date_epoch_from_env(void)
+{
+	/* The compiler is recent enough to handle SOURCE_DATE_EPOCH itself
+	 * so we do not need to do anything here.
+	 */
+	return false;
+}
+#endif
 
 int main(int argc, char **argv)
 {

toolchain/toolchain-wrapper.mk

@@ -28,6 +28,9 @@ TOOLCHAIN_WRAPPER_OPTS += -ffile-prefix-map=$(BASE_DIR)=buildroot
 else
 TOOLCHAIN_WRAPPER_OPTS += -D__FILE__=\"\" -D__BASE_FILE__=\"\" -Wno-builtin-macro-redefined
 endif
+ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_7),)
+TOOLCHAIN_WRAPPER_OPTS += -DBR_NEED_SOURCE_DATE_EPOCH
+endif
 endif
 
 # We create a list like '"-mfoo", "-mbar", "-mbarfoo"' so that each flag is a