Ask pkg-config to print all paths to find fontconfig

When "fontconfig/fontconfig.h" is in a standard location (like
/usr/include), pkg-config does not include the corresponding "-I" flag,
unless explicitly requested by the PKG_CONFIG_ALLOW_SYSTEM_CFLAGS
environment variable.

This patch also uses regular expressions to split the different "-I"
components, which are more robust than fixed-length substrings (esp.
towards then end of the string).