Never overwrite .c files when re2c does not exist.

configure.in

@@ -127,6 +127,8 @@ dnl AC_PROG_CXXCPP
 dnl check for -R, etc. switch
 PHP_RUNPATH_SWITCH
 
+AC_CHECK_PROG(RE2C, re2c, re2c, [exit 0;])
+
 AC_PROG_RANLIB
 AC_PROG_LN_S
 AC_PROG_AWK
@@ -1071,6 +1073,7 @@ PHP_SUBST(PHP_RPATHS)
 PHP_SUBST(PHP_SAPI)
 PHP_SUBST_OLD(PHP_VERSION)
 PHP_SUBST(PROG_SENDMAIL)
+PHP_SUBST(RE2C)
 PHP_SUBST(SHELL)
 PHP_SUBST(SHARED_LIBTOOL)
 PHP_SUBST(TSRM_DIR)

ext/standard/Makefile.frag

@@ -3,9 +3,9 @@ $(srcdir)/parsedate.c: $(srcdir)/parsedate.y
 	$(YACC) $(srcdir)/parsedate.y -o $@
 
 $(srcdir)/var_unserializer.c: $(srcdir)/var_unserializer.re
-	re2c -b $(srcdir)/var_unserializer.re > $@
+	$(RE2C) -b $(srcdir)/var_unserializer.re > $@
 
 $(srcdir)/url_scanner_ex.c: $(srcdir)/url_scanner_ex.re
-	re2c -b $(srcdir)/url_scanner_ex.re > $@
+	$(RE2C) -b $(srcdir)/url_scanner_ex.re > $@
 
 $(srcdir)/info.c: $(builddir)/../../main/build-defs.h