[...]
g++ -DPACKAGE_NAME=\"Gource\" -DPACKAGE_TARNAME=\"gource\"
-DPACKAGE_VERSION=\"0.54\" -DPACKAGE_STRING=\"Gource\ 0.54\" 
-DPACKAGE_BUGREPORT=\"acaudwell@gmail.com\" -DPACKAGE_URL=\"\" 
-DPACKAGE=\"gource\" -DVERSION=\"0.54\" -DHAVE_TIMEGM=1 
-DHAVE_UNSETENV=1 -DHAVE_STDIO_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 
-DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_STRINGS_H=1 
-DHAVE_SYS_STAT_H=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_UNISTD_H=1 
-DSTDC_HEADERS=1 -DHAVE_GL_GL_H=1 -DHAVE_GL=1 -DHAVE_GL_GLU_H=1 
-DHAVE_GLU=1 -DHAVE_IMG_LOADPNG_RW=1 -DHAVE_IMG_LOADJPG_RW=1 
-DHAVE_BOOST=/\*\*/ -DHAVE_BOOST_SYSTEM=/\*\*/ 
-DHAVE_BOOST_FILESYSTEM=/\*\*/ -DHAVE_LIBTINYXML=1 -I. 
-DSDLAPP_RESOURCE_DIR=\"/usr/share/gource\" 
-DSDLAPP_FONT_DIR=\"/usr/share/fonts/truetype/freefont\" -Wdate-time 
-D_FORTIFY_SOURCE=3   -I/usr/include/freetype2 -I/usr/include/libpng16 
  -I/usr/include/SDL2 -D_REENTRANT -I/usr/include/libpng16
-I/usr/include/x86_64-linux-gnu -I/usr/include/webp 
-I/usr/include/libpng16  -I/usr/include -std=gnu++0x -Wall 
-Wno-sign-compare -Wno-reorder -Wno-unused-but-set-variable 
-Wno-unused-variable -g -O2 -fno-omit-frame-pointer 
-mno-omit-leaf-frame-pointer -ffile-prefix-map=/<<PKGBUILDDIR>>=. 
-flto=auto -ffat-lto-objects -fstack-protector-strong 
-fstack-clash-protection -Wformat -Werror=format-security 
-fcf-protection 
-fdebug-prefix-map=/<<PKGBUILDDIR>>=/usr/src/gource-0.54-1build5   -c -o 
src/gource-key.o `test -f 'src/key.cpp' || echo './'`src/key.cpp
src/gource_settings.cpp: In member function ‘void
GourceSettings::importGourceSettings(ConfFile&, ConfSection*)’:
src/gource_settings.cpp:836:36: error: ‘extension’ was not declared in
this scope
   836 |             std::string file_ext = extension(p);
       |                                    ^~~~~~~~~