добавил в параметры компилятора библиотеку curl
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g++ -IC:\qt4\minGW\include\curl -O0 -g3 -Wall -c -fmessage-length=0 -DCURL_STATICLIB -g -Lcurl/lib -omain.o ..\main.cpp
g++ -LC:\qt4\minGW\lib -ocurl.exe main.o -lcurl

C:\qt4\minGW\lib/libcurl.a(easy.o):(.text+0x7e): undefined reference to `WSAStartup@8'
C:\qt4\minGW\lib/libcurl.a(easy.o):(.text+0x9b): undefined reference to `WSACleanup@0'
C:\qt4\minGW\lib/libcurl.a(easy.o):(.text+0x188): undefined reference to `WSACleanup@0'
C:\qt4\minGW\lib/libcurl.a(sendf.o):(.text+0xa1): undefined reference to `recv@16'
C:\qt4\minGW\lib/libcurl.a(sendf.o):(.text+0xb9): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(sendf.o):(.text+0x73d): undefined reference to `send@16'
C:\qt4\minGW\lib/libcurl.a(sendf.o):(.text+0x758): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0xf3): undefined reference to `ioctlsocket@12'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x193): undefined reference to `getsockopt@20'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x1c7): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x206): undefined reference to `setsockopt@20'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x45e): undefined reference to `socket@12'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x4f4): undefined reference to `setsockopt@20'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x5fe): undefined reference to `bind@12'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x63f): undefined reference to `ntohs@4'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x709): undefined reference to `closesocket@4'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x73b): undefined reference to `connect@12'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x7c9): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x8a9): undefined reference to `getsockname@12'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x8e0): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x92c): undefined reference to `htons@4'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x94b): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0x97d): undefined reference to `WSAGetLastError@0'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0xae0): undefined reference to `htons@4'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0xb18): undefined reference to `closesocket@4'
C:\qt4\minGW\lib/libcurl.a(connect.o):(.text+0xcbd): undefined reference to `WSAGetLastError@0'