common/environment: empty file before writing.

Emptying the headerfile before writing into it. This prevents
defining/undefining a macro multiple times.

addresses #3765.

common/environment/configure/timestamp-macros.sh

@@ -1,6 +1,7 @@
 if [ -n "$SOURCE_DATE_EPOCH" ]; then
 	CFLAGS+=" -Wno-builtin-macro-redefined -include$XBPS_STATEDIR/timestamp-macros.h"
 	CXXFLAGS+=" -Wno-builtin-macro-redefined -include$XBPS_STATEDIR/timestamp-macros.h"
+	printf > $XBPS_STATEDIR/timestamp-macros.h
 	for i in "DATE,%b %d %Y" "TIME,%H:%M:%S" "DATETIME,%b %d %Y %H:%M:%S"; do
 		mcr=${i%%,*}
 		val=$(LC_ALL=C date --date "@$SOURCE_DATE_EPOCH" +"${i#*,}")