graphics/librasterlite2: Explicitly link with libm and fix build after base r345349

ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to floor
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to atan2
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to cos
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to sin
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to tan
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to atan
ld: error: /wrkdirs/usr/ports/graphics/librasterlite2/work/librasterlite2-1.0.0-rc0/src/.libs/librasterlite2.so: undefined reference to pow

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p501065_s347375/logs/errors/librasterlite2-1.0.0_4.log

https://www.gaia-gis.it/fossil/librasterlite2/info/961e1780c4f016bc