Make blows up with missing curl/curl.h

[Rytiss]

Describe the bug
While compiling server with ./configure --disable-client --disable-manager --enable-fcgi && make on a system that does not have libcurl installed, configure does not catch the missing dependency and the build blows up.

  CXX      remote_submit.o
../lib/remote_submit.cpp:24:10: fatal error: curl/curl.h: No such file or directory
 #include <curl/curl.h>
          ^~~~~~~~~~~~~
compilation terminated.
make[2]: *** [Makefile:820: remote_submit.o] Error 1

Steps To Reproduce

git clone https://github.com/BOINC/boinc.git boinc-git
cd boinc-git
git checkout tags/server_release/1.2/1.2.1 -b server_release/1.2.1
./_autosetup
./configure --disable-client --disable-manager --enable-fcgi
make

Expected behavior
Configure should notify about the missing lib and abort.