BUILD: improve the makefile's support for libpcre

BUILD: improve the makefile's support for libpcre

Currently when cross-compiling, it's generally necessary to force
PCREDIR which the Makefile automatically appends /include and /lib to.
Unfortunately on most 64-bit linux distros, the lib path is instead
/lib64, which is really annoying to fix in the makefile.

So now we're computing PCRE_INC and PCRE_LIB from PCREDIR and using
these ones instead. If one wants to force paths individually, it is
possible to set them instead of setting PCREDIR. The old behaviour
of not passing anything to the compiler when PCREDIR is forced to blank
is conserved.