Hmm, the log seems to be filled with a zillion errors starting with:
Status: Preprocessing /tmp/tmpsoOT1L.h
Status: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include -U __BLOCKS__ -U __GNUC__ -dD "-Dinline=" "-D__inline__=" "-D__extension__=" "-D_Bool=uint8_t" "-D__const=const" "-D__asm__(x)=" "-D__asm(x)=" "-DCTYPESGEN=1" "-D__uint16_t=uint16_t" "-D__uint32_t=uint32_t" "-D__uint64_t=uint64_t" /tmp/tmpsoOT1L.h
Error: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include: In file included from /usr/include/stdio.h:64,
Error: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include: from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include/grass/gis.h:24,
Error: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include: from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include/grass/stats.h:5,
Error: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include: from /tmp/tmpsoOT1L.h:1:
Error: /usr/bin/llvm-gcc-4.2 -E -I/opt/local/include -DPACKAGE="grasslibs" -DPACKAGE="grasslibs" -I/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_gis_grass/grass/work/grass-6.4.2/dist.i386-apple-darwin12.2.0/include: /usr/include/sys/cdefs.h:81:2: warning: #warning "Unsupported compiler detected"
Curious that it says llvm-gcc-4.2 is not a supported compiler; we previously switched the port from clang to llvm-gcc-4.2; see #32790.
Can you try updating to Xcode 4.6? Also ensure the Xcode command line tools are up to date (open Xcode, go to Preferences > Downloads and install or update the command line tools from there).
Note that even once you get past this error, grass still won't build with tcl / tk 8.6; see #37578.
