Make BUILD_SHARED_LIBS flag configurable

Preparing for static builds, which can be done without breaching
licensing issues with Go transport enabled.

Signed-off-by: Hidde Beydals <hello@hidde.co>

Author: hiddeco

hack/Makefile

@@ -17,6 +17,7 @@ BUILD_TYPE ?= "RelWithDebInfo"
 FLAGS ?=
 USE_HTTPS ?= OFF
 USE_SSH ?= OFF
+BUILD_SHARED_LIBS ?= ON
 
 # Cmake version to be installed.
 CMAKE_VERSION ?= 3.21.3
@@ -119,7 +120,7 @@ $(LIBGIT2):
 		-DCMAKE_INSTALL_LIBDIR:PATH=$(INSTALL_LIBDIR) \
 		-DBUILD_CLAR:BOOL:BOOL=OFF \
 		-DTHREADSAFE:BOOL=ON \
-		-DBUILD_SHARED_LIBS=ON \
+		-DBUILD_SHARED_LIBS:BOOL=$(BUILD_SHARED_LIBS) \
 		-DUSE_BUNDLED_ZLIB:BOOL=OFF \
 		-DUSE_HTTP_PARSER:STRING=builtin  \
 		-DREGEX_BACKEND:STRING=builtin \