Makefile: use curl-config to determine curl flags

curl-config should always be installed alongside a curl distribution,
and its purpose is to provide flags for building against libcurl, so
use it instead of guessing flags and dependent libraries.

Allow overriding CURL_CONFIG to a custom path to curl-config, to
compile against a curl installation other than the first in PATH.

Depending on the set of features curl is compiled with, there may be
more libraries required than the previous two options of -lssl and
-lidn. For example, with a vanilla build of libcurl-7.36.0 on Mac OS X
10.9:

$ ~/d/curl-out-7.36.0/lib/curl-config --libs
-L/Users/dborowitz/d/curl-out-7.36.0/lib -lcurl -lgssapi_krb5 -lresolv -lldap -lz

Use this only when CURLDIR is not explicitly specified, to continue
supporting older builds.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

diff --git a/Makefile b/Makefile
index 2128ce3..d6330bc 100644
--- a/Makefile
+++ b/Makefile
@@ -34,8 +34,12 @@
 # git-http-push are not built, and you cannot use http:// and https://
 # transports (neither smart nor dumb).
 #
+# Define CURL_CONFIG to the path to a curl-config binary other than the
+# default 'curl-config'.
+#
 # Define CURLDIR=/foo/bar if your curl header and library files are in
-# /foo/bar/include and /foo/bar/lib directories.
+# /foo/bar/include and /foo/bar/lib directories.  This overrides CURL_CONFIG,
+# but is less robust.
 #
 # Define NO_EXPAT if you do not have expat installed.  git-http-push is
 # not built, and you cannot push using http:// and https:// transports (dumb).
@@ -143,9 +147,11 @@
 #
 # Define NEEDS_SSL_WITH_CRYPTO if you need -lssl when using -lcrypto (Darwin).
 #
-# Define NEEDS_SSL_WITH_CURL if you need -lssl with -lcurl (Minix).
+# Define NEEDS_SSL_WITH_CURL if you need -lssl with -lcurl (Minix).  Only used
+# if CURLDIR is set.
 #
-# Define NEEDS_IDN_WITH_CURL if you need -lidn when using -lcurl (Minix).
+# Define NEEDS_IDN_WITH_CURL if you need -lidn when using -lcurl (Minix).  Only
+# used if CURLDIR is set.
 #
 # Define NEEDS_LIBICONV if linking with libc is not enough (Darwin).
 #
@@ -1121,17 +1127,22 @@
 		# Try "-Wl,-rpath=$(CURLDIR)/$(lib)" in such a case.
 		BASIC_CFLAGS += -I$(CURLDIR)/include
 		CURL_LIBCURL = -L$(CURLDIR)/$(lib) $(CC_LD_DYNPATH)$(CURLDIR)/$(lib) -lcurl
-	else
-		CURL_LIBCURL = -lcurl
-	endif
-	ifdef NEEDS_SSL_WITH_CURL
-		CURL_LIBCURL += -lssl
-		ifdef NEEDS_CRYPTO_WITH_SSL
-			CURL_LIBCURL += -lcrypto
+		ifdef NEEDS_SSL_WITH_CURL
+			CURL_LIBCURL += -lssl
+			ifdef NEEDS_CRYPTO_WITH_SSL
+				CURL_LIBCURL += -lcrypto
+			endif
 		endif
-	endif
-	ifdef NEEDS_IDN_WITH_CURL
-		CURL_LIBCURL += -lidn
+		ifdef NEEDS_IDN_WITH_CURL
+			CURL_LIBCURL += -lidn
+		endif
+	else
+		CURL_CONFIG ?= curl-config
+		BASIC_CFLAGS += $(shell $(CURL_CONFIG) --cflags)
+		CURL_LIBCURL = $(shell $(CURL_CONFIG) --libs)
+		ifeq "$(CURL_LIBCURL)" ""
+			$(error curl not detected; try setting CURLDIR)
+		endif
 	endif
 
 	REMOTE_CURL_PRIMARY = git-remote-http$X