www/varnish7: Use upstream release archive and some improvements to port

• Use upstream release archive as recommended by USE_GITHUB section in Porters Handbook
• Remove USES= compiler as there's no need to modify configure script
• Remove unneeded gmake and readline dependency definitions
• Use USES= localbase instead of CFLAGS variable
• Remove REINPLACE_CMD for GCC
• Rearrange Makefile to follow Porters Handbook more closely and improve readability

PR: 266467
Reviewed by: dbaio (maintainer)