Fix convert_to_utf8 failing on systems where python3 is not in /usr/bin

scripts/convert_to_utf8 had a hardcoded #!/usr/bin/python3 shebang which
fails on FreeBSD and other systems where Python 3 lives in /usr/local/bin.

- Change shebang to #!/usr/bin/env python3 for portability
- Add PYTHON= @python@ to templates/Makefile.in and invoke the script via
  $(PYTHON) so the build uses the exact interpreter detected by configure,
  consistent with all other Makefiles in the tree

Author: thegushi

scripts/convert_to_utf8

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import os
 import sys

templates/Makefile.in

@@ -31,6 +31,7 @@ CC=		@CC@
 CHMOD=  	@CHMOD@
 INSTALL=	@INSTALL@
 TRUE=		@TRUE@
+PYTHON=		@PYTHON@
 
 DEFS=   	@DEFS@
 
@@ -61,7 +62,7 @@ all: converttemplates
 
 # Use a stamp file to track conversion, so it only happens once
 .converted.stamp:
-	../scripts/convert_to_utf8 -d .
+	$(PYTHON) ../scripts/convert_to_utf8 -d .
 	touch .converted.stamp
 
 converttemplates: .converted.stamp