www/nghttp2: Update to version 1.7.1

• Convert MASTER_SITES to USE_GITHUB
  • Remove USES=tar:xz
• Remove unneeded RUN_DEPENDS
• Add USES=autoreconf
• Fix pkgconfig path
• Remove defunct configure jemalloc fix
• Fix LDFLAGS -ldl problem

Reviewed by: koobs (mentor), feld (mentor)
Approved by: feld (mentor)
Differential Revision: D5218
MFH: 2016Q1