I know there is mis-match of fortran compiler for C++ compiler.
I'm not clear what you're asking, and you don't provide example error
If you have a mis-match between the compilers, then you have
to fix it, and figure out how to link code created by the 2 compilers.
That's what the example Makefiles provided do.
please note that g77 is considered obsolete.
also, the gcc 3.x series is _very_ old. are you
sure that when you compile the reax library with
g77, that you also compile the rest of lammps with
the matching 3.x gcc compiler?
for using g77, you may need to use
-D_F2C_LINUX when compiling the c++ part of lammps.
gcc 4.x includes gfortran and reax compiles
correctly out of the box with reasonably recent gfortran
versions and are thus recommended. you are making
your own life needlessly complicated by using