The compiler indicates a "Warning" due to the fact the library function:
char * strstr (const char * s1, const char * s2)
expects both parameters passed to it to be of type "const char *", which obviously:
static char rxBuff[250];
is not "const", however this should not prevent the program from being compiled successfully.
The latest version of the PICC18 Compiler appears to no longer issue the "Warning".
You could cast rxBuff:
p_poc = strstr((const char *)rxBuff, "+CMGR: \"REC");
or upgrade to the latest version of PICC18, I'm current using version 9.66 and no warning is issued.
Hope this info helps in your endeavors.