configure: add switches to set search paths

configure: add switches to set search paths

   --sysincludepaths=.. specify system include paths, colon separated"
     Sets CONFIG_TCC_SYSINCLUDEPATHS

   --libpaths=...       specify system library paths, colon separated"
     Sets CONFIG_TCC_LIBPATHS

   --crtprefix=...      specify location of crt?.o"
     Sets CONFIG_TCC_CRTPREFIX

   --elfinterp=...      specify elf interpreter"
     Sets CONFIG_TCC_ELFINTERP

Also the CONFIG_TCC_XXX were renamed to make them look
more consistent.

Also move the elf_interp definitions to tcc.h.