Merge branch 'jk/build-with-right-curl'

The build procedure did not use the libcurl library and its include
files correctly for a custom-built installation.

* jk/build-with-right-curl:
  Makefile: avoid running curl-config unnecessarily
  Makefile: use curl-config --cflags
  Makefile: avoid running curl-config multiple times

diff --cc Makefile
Simple merge