add default pkg_config_path

[kazeburo]

Failed install rrdtool on CentOS6 with this error.

checking for pango_cairo_context_set_font_options in -lpangocairo-1.0... yes
checking pango/pango.h usability... no
checking pango/pango.h presence... yes
configure: WARNING: pango/pango.h: present but cannot be compiled
configure: WARNING: pango/pango.h:     check for missing prerequisite headers?
configure: WARNING: pango/pango.h: see the Autoconf documentation
configure: WARNING: pango/pango.h:     section "Present But Cannot Be Compiled"
configure: WARNING: pango/pango.h: proceeding with the compiler's result
checking for pango/pango.h... no

CentOS6's pkg_config_path is /usr/lib64/pkgconfig

$ find /usr -name 'pango.pc'
/usr/lib64/pkgconfig/pango.pc

This p-r add /usr/lib64/pkgconfig to default pkg_config_path of MyBuilder.pm

[gfx]

LGTM. Now you are a maintainer of Alien::RRDtool, so please release a new version.

[kazeburo]

Just released 0.06 https://metacpan.org/release/KAZEBURO/Alien-RRDtool-0.06
Thank you!