Can't be compiled with clang

[jelu]

# Compiling >>>grafix/Font5.cpp<<<
clang++ -Wall -O2 -o release/gfx_Font5.o -c ./src/grafix/Font5.cpp -I./include -DPPL7LIB  -I/usr/local/include  -DHAVE_CONFIG_H  -pthread 	-I/usr/include     -I/usr/include/mysql   	-I/usr/include 	    	 	  	-I/usr/include -I/usr/include/freetype2 -I/usr/include/libpng16   
./src/grafix/Font5.cpp:251:6: error: functions that differ only in their return
      type cannot be overloaded
void BltGlyph_M8_32 (GLYPH *g)
~~~~ ^
./src/grafix/Font5.cpp:58:6: note: previous declaration is here
        int BltGlyph_M8_32 (GLYPH *g);
        ~~~ ^
1 error generated.
Makefile:1103: recipe for target 'release/gfx_Font5.o' failed
make: *** [release/gfx_Font5.o] Error 1
The command "make" exited with 2.

Full output https://travis-ci.com/DNS-OARC/dnsmeter/jobs/229599953#L1011