Use the static libgcc option only for Windows.

For obvious reasons, the -static-libgcc linker option is not supported
by non gcc compilers, like clang. Since we only need this option when
building Windows DLL's, we can easily avoid the problem with a
conditional on the mingw compiler.

src/Makefile.am

@@ -6,10 +6,13 @@ lib_LTLIBRARIES = libdivecomputer.la
 libdivecomputer_la_LIBADD = $(LIBUSB_LIBS)
 libdivecomputer_la_LDFLAGS = \
 	-version-info $(DC_VERSION_LIBTOOL) \
-	-Wc,-static-libgcc \
 	-no-undefined \
 	-export-symbols libdivecomputer.exp
 
+if OS_WIN32
+libdivecomputer_la_LDFLAGS += -Wc,-static-libgcc
+endif
+
 libdivecomputer_la_SOURCES = \
 	version.c \
 	descriptor.c \