Makefile.am: Fix unportable test(1) operator. (#459)

file: Makefile.am

Notes:
The POSIX comparison operator for test(1) is =; bash supports == but not even test from GNU coreutils does.

Credit:
Thomas Klausner

Makefile.am

@@ -119,7 +119,7 @@ $(DSP): win32/msvcproj.head win32/msvcproj.foot Makefile.am
 	for file in $$sorted_hdrs; do \
 		echo "# Begin Source File"; \
 		echo ""; \
-		if [ "$$file" == "libssh2_config.h" ]; \
+		if [ "$$file" = "libssh2_config.h" ]; \
 		then \
 			echo "SOURCE=.\\"$$file; \
 		else \