Missing function definitions with libcurl-devel on CentOS 6.6 - c++

Question

I am having trouble compiling my program which uses libcurl.
The environment I am using is CentOS 6.6 on a VM, and I am getting errors which suggests that the library has either:
not installed correctly or
cannot be found.
I did a
sudo yum install libcurl followed by a
sudo yum install libcurl-devel followed by a
sudo yum update
It seems that the curl.h has been put under /usr/include/curl which seems correct so I am unsure as to why this has occurred.
It is worth noting that a test program with #include <curl/curl.h> is compiling.
Also, when libcurl-devel is not installed (and only libcurl is),
gcc complained it couldn't find curl.h, as expected,
there was also no curl directory under /usr/include/, as expected,
however libcurl did install a curl file under /usr/bin/, as expected.
In Ubuntu I only had to install libcurl-openssl
Some example gcc errors:
$gcc new.c -lcurl
new.c: In function ‘main’:
new.c:74: error: ‘CURLOPT_MAIL_FROM’ undeclared (first use in this function)
new.c:74: error: (Each undeclared identifier is reported only once
new.c:74: error: for each function it appears in.)
new.c:81: error: ‘CURLOPT_MAIL_RCPT’ undeclared (first use in this function)

Answer 1

If you are installing libcurl-devel from the CentOS RPM then it absolutely should not be putting the header, or anything else, in /usr/local/curl. /usr/local is for locally-built components, not for components provided by the OS. Exactly what that means can be a subject for debate, but I can confirm that the CentOS 6 RPM for libcurl-devel installs the headers in /usr/include/curl.
With that being the case, if CentOS's libcurl-devel is installed, then you shouldn't need any special compiler flags to get the compiler to resolve
#include <curl/curl.h>
correctly, but if you happen to have a different curl.h somewhere on your system then you could cause it to be chosen instead. Still, although I could maybe believe the compiler finding /usr/local/include/curl/curl.h automatically (that is, without -I/usr/local/include), I would be very surprised to hear that it finds /usr/local/curl/curl.h automatically.
But that probably isn't your problem. CentOS 6 provides version 7.19 of libcurl. That version does not provide the macros you are trying to use, at least not in libcurl-devel. I have checked for a possible add-on package corresponding to Ubuntu's libcurl-openssl, but there is no CentOS 6 package (or EPEL/el6 package) that has both "curl" and "ssl" in its name.
If you can identify what minimum version of libcurl you need, you can probably build it yourself. In such cases I prefer to take a source RPM from Fedora's Koji system, build RPMs from it on the local system, and install them (possibly updating existing RPMs in the process). Alternatively, you can probably build and use a private copy, but in that case you will want to link it statically into your program.