Add a configure option to disable pcre JIT compilation

Add a configure option to disable pcre JIT compilation

While JIT compilation makes filtering faster it can
cause false-positive valgrind complaints like:

    ==94928== Thread 2:
    ==94928== Conditional jump or move depends on uninitialised value(s)
    ==94928==    at 0x40A990B: ???
    ==94928==    by 0x955E761: ???
    ==94928==  Uninitialised value was created by a heap allocation
    ==94928==    at 0x4C26A44: malloc (in /usr/local/lib/valgrind/vgpreload_memcheck-amd64-freebsd.so)
    ==94928==    by 0x5114247: BUF_MEM_grow_clean (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x50F2FD2: ??? (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x50EDB7F: ??? (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x50ECD78: ??? (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x50ECC75: BIO_write (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x5C15B0F: ??? (in /usr/local/lib/libssl.so.11)
    ==94928==    by 0x5C422A9: ??? (in /usr/local/lib/libssl.so.11)
    ==94928==    by 0x5C39156: ??? (in /usr/local/lib/libssl.so.11)
    ==94928==    by 0x5C07F9A: ??? (in /usr/local/lib/libssl.so.11)
    ==94928==    by 0x50ED3AA: BIO_ctrl (in /usr/local/lib/libcrypto.so.11)
    ==94928==    by 0x460033: create_server_ssl_connection (openssl.c:1150)

As reported by Gwyn Ciesla in SF bug 924 it also can
cause problems when the SELinux policy does not grant
Privoxy "execmem" privileges.